BuddyPress.org

If you make a new group and join user(s) into it, does this problem reoccur? If it does, try to deactivate all plugins other than BuddyPress and make a new group and try again. If it doesn’t reoccur, then it’s a conflict with a plugin.

I have read recently of problems with feedwordpress + BuddyPress, so there’s an incompatibility somewhere (if indeed feedwordpress works with WPMU).

I’m inclined to thing this is a problem from the Groupblogs plugin, but let me know what happens when you test the above.