I was added to a group but a few months later removed myself from the group via my phone. Punching too many "yes" answers I hit the button that said "prevent anyone from adding you to this group again". So now I want to be in that group again and I don't know how....
I was in a similar situation once. The way I solved this problem was by asking the group admin to copy and send me the full URL that appears when s/he opens the facebook group page (excluding strings like &view=doc if any). Example: