add and remove users from groups

bookwyrm/models/group.py

@@ -42,4 +42,11 @@ class GroupMember(models.Model):
     """Users who are members of a group"""
 
     group = models.ForeignKey("Group", on_delete=models.CASCADE)
-    user = models.ForeignKey("User", on_delete=models.CASCADE)
+    user = models.ForeignKey("User", on_delete=models.CASCADE)
+
+    class Meta:
+        constraints = [
+          models.UniqueConstraint(
+            fields=["group", "user"], name="unique_member"
+          )
+        ]