Happens to me when OTHER people join the group I'm in.
(Ex.: Person #1 invites me (#2) and then #1 invites #3 and me (#2) can't see #3, but everyone else can.)
To fix this, I just tell the person that I can't see to leave the group and get invited again. It always shows the second time.
Oh, and I too, believe it has nothing to do with Addons.