It could be. As an administrator I can modify users/recipients of discussion (only if administrator is creator/author of discussion). Tried to recreate scenario when user "A" creates private discussion, and adds user "B" as a recipient/participant. Discussion is created without any problem. But, problem happens when user "A" or "B" wants do add or remove users "A", "B", or "C"... All users ("A", "B" and "C" have correct permissions - ability to create, modify private discussion and to view user list).
