diff --git a/core/api/api.go b/core/api/api.go
index aa1b835..d79d04e 100644
--- a/core/api/api.go
+++ b/core/api/api.go
@@ -290,3 +290,36 @@ func RequestTokenHasUserID(c *gin.Context, userID string) bool {
 func RequestUserIsAdmin(c *gin.Context) bool {
 	return service.IsAdmin(GetRequestTokenEntityID(c))
 }
+
+// RequestUserIsGroupOwner reports whether the bearer's subject entity
+// is on the GroupOwner roster for groupID. Used by gates that let the
+// owners of a group manage its members and join requests without
+// requiring global admin elevation. Returns false for unauth'd
+// requests and on any lookup error.
+func RequestUserIsGroupOwner(c *gin.Context, groupID string) bool {
+	entityID := GetRequestTokenEntityID(c)
+	if entityID == "" {
+		return false
+	}
+	owner, err := service.GetGroupOwner(groupID, entityID)
+	return err == nil && owner.EntityID != ""
+}
+
+// requireGroupOwnerOrAdmin is the standard gate for endpoints that
+// mutate group state — members, owners, join requests, conditional
+// bindings. Group owners can manage their own group; admins override;
+// sentinel:all bypasses (matches the codebase-wide convention where
+// the scope is reserved for first-party internal automation). Aborts
+// with 403 on failure and returns false; otherwise returns true and
+// the caller continues.
+func requireGroupOwnerOrAdmin(c *gin.Context, groupID string) bool {
+	if Any(
+		RequestTokenHasScope(c, "sentinel:all"),
+		RequestUserIsGroupOwner(c, groupID),
+		RequestUserIsAdmin(c),
+	) {
+		return true
+	}
+	c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "you are not authorized to manage this group"})
+	return false
+}
diff --git a/core/api/conditional_binding.go b/core/api/conditional_binding.go
index b6af2f1..a3e8982 100644
--- a/core/api/conditional_binding.go
+++ b/core/api/conditional_binding.go
@@ -25,6 +25,9 @@ type createConditionalBindingRequest struct {
 
 func CreateGroupConditionalBinding(c *gin.Context) {
 	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	var req createConditionalBindingRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
 		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
@@ -57,6 +60,9 @@ func CreateGroupConditionalBinding(c *gin.Context) {
 
 func DeleteGroupConditionalBinding(c *gin.Context) {
 	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	bindingID := c.Param("bindingID")
 	if err := service.DeleteConditionalBinding(id, bindingID); err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
diff --git a/core/api/group.go b/core/api/group.go
index 1306dee..cad20ab 100644
--- a/core/api/group.go
+++ b/core/api/group.go
@@ -128,6 +128,17 @@ func CreateOrUpdateGroup(c *gin.Context) {
 		return
 	}
 
+	// Create vs. update have different trust requirements. Anyone with a
+	// valid bearer can create a group (they become the auto-added owner
+	// below). Updates have to clear the owner-or-admin gate against the
+	// existing group — without this check, anyone could rename or
+	// rewrite allowed_sources on any group, including the Admins group.
+	if existing.ID == "" {
+		Require(c, RequestTokenExists(c))
+	} else if !requireGroupOwnerOrAdmin(c, existing.ID) {
+		return
+	}
+
 	available, err := service.IsGroupNameAvailable(req.Name, existing.ID)
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
@@ -207,6 +218,9 @@ func DeleteGroup(c *gin.Context) {
 		c.JSON(http.StatusForbidden, gin.H{"error": "the Admins group cannot be deleted"})
 		return
 	}
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	if err := service.DeleteGroup(id); err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 		return
@@ -236,6 +250,9 @@ type addGroupMemberRequest struct {
 
 func AddGroupMember(c *gin.Context) {
 	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	var req addGroupMemberRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
 		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
@@ -266,6 +283,9 @@ func AddGroupMember(c *gin.Context) {
 
 func RemoveGroupMember(c *gin.Context) {
 	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	entityID := c.Param("entityID")
 	source := c.Query("source")
 	if err := service.DeleteGroupMember(id, entityID, source); err != nil {
@@ -296,6 +316,9 @@ type addGroupOwnerRequest struct {
 
 func AddGroupOwner(c *gin.Context) {
 	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	var req addGroupOwnerRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
 		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
@@ -315,6 +338,9 @@ func AddGroupOwner(c *gin.Context) {
 
 func RemoveGroupOwner(c *gin.Context) {
 	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	entityID := c.Param("entityID")
 	if err := service.DeleteGroupOwner(id, entityID); err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
@@ -362,6 +388,15 @@ func CreateGroupJoinRequest(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
 		return
 	}
+	// Users request to join groups on their own behalf — the bearer's
+	// entity must match the requested entity_id. The only way around
+	// that is admin or internal; group owners can't backdoor people in
+	// via this endpoint (they'd use AddGroupMember directly).
+	Require(c, Any(
+		RequestTokenHasScope(c, "sentinel:all"),
+		RequestTokenHasEntityID(c, req.EntityID),
+		RequestUserIsAdmin(c),
+	))
 	if err := validateMembershipExpiration(req.HasExpiration, req.ExpiresAt); err != nil {
 		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
 		return
@@ -397,6 +432,10 @@ type reviewJoinRequestRequest struct {
 }
 
 func ApproveGroupJoinRequest(c *gin.Context) {
+	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	requestID := c.Param("requestID")
 	var req reviewJoinRequestRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
@@ -452,6 +491,10 @@ func ApproveGroupJoinRequest(c *gin.Context) {
 }
 
 func RejectGroupJoinRequest(c *gin.Context) {
+	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	requestID := c.Param("requestID")
 	var req reviewJoinRequestRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
@@ -479,6 +522,10 @@ func RejectGroupJoinRequest(c *gin.Context) {
 }
 
 func DeleteGroupJoinRequest(c *gin.Context) {
+	id := c.Param("id")
+	if !requireGroupOwnerOrAdmin(c, id) {
+		return
+	}
 	requestID := c.Param("requestID")
 	if err := service.DeleteJoinRequest(requestID); err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
@@ -495,12 +542,24 @@ type createJoinRequestCommentRequest struct {
 }
 
 func CreateJoinRequestComment(c *gin.Context) {
+	id := c.Param("id")
 	requestID := c.Param("requestID")
 	var req createJoinRequestCommentRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
 		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
 		return
 	}
+	// Comments are scoped to the join-request thread: the requester
+	// (commenting on their own request) and the group's owners /
+	// admins (reviewing the request) are the legitimate posters.
+	// Bearer must match the comment's claimed entity_id; the owner/
+	// admin path bypasses the self check.
+	Require(c, Any(
+		RequestTokenHasScope(c, "sentinel:all"),
+		RequestTokenHasEntityID(c, req.EntityID),
+		RequestUserIsGroupOwner(c, id),
+		RequestUserIsAdmin(c),
+	))
 	comment, err := service.CreateJoinRequestComment(model.GroupJoinRequestComment{
 		RequestID: requestID,
 		EntityID:  req.EntityID,
@@ -514,7 +573,26 @@ func CreateJoinRequestComment(c *gin.Context) {
 }
 
 func DeleteJoinRequestComment(c *gin.Context) {
+	id := c.Param("id")
 	commentID := c.Param("commentID")
+	// Look up the comment first so we can authorize against its
+	// claimed author (the entity who posted it can delete their own
+	// comment; otherwise owner/admin/internal).
+	comment, err := service.GetJoinRequestComment(commentID)
+	if err != nil {
+		if err == gorm.ErrRecordNotFound {
+			c.JSON(http.StatusNotFound, gin.H{"error": "comment not found"})
+			return
+		}
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+	Require(c, Any(
+		RequestTokenHasScope(c, "sentinel:all"),
+		RequestTokenHasEntityID(c, comment.EntityID),
+		RequestUserIsGroupOwner(c, id),
+		RequestUserIsAdmin(c),
+	))
 	if err := service.DeleteJoinRequestComment(commentID); err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 		return
diff --git a/core/service/group.go b/core/service/group.go
index a274127..2567000 100644
--- a/core/service/group.go
+++ b/core/service/group.go
@@ -254,6 +254,17 @@ func GetCommentsForJoinRequest(requestID string) ([]model.GroupJoinRequestCommen
 	return comments, nil
 }
 
+// GetJoinRequestComment returns a single comment by ID. Used by the
+// delete handler to authorize the requester against the comment's
+// claimed author before letting them delete it.
+func GetJoinRequestComment(id string) (model.GroupJoinRequestComment, error) {
+	var comment model.GroupJoinRequestComment
+	if err := database.DB.Where("id = ?", id).First(&comment).Error; err != nil {
+		return model.GroupJoinRequestComment{}, err
+	}
+	return comment, nil
+}
+
 func CreateJoinRequestComment(comment model.GroupJoinRequestComment) (model.GroupJoinRequestComment, error) {
 	if comment.ID == "" {
 		comment.ID = ulid.Make().Prefixed("gjrc")