When we were doing tiger before, we invited waveforce to party and entered the dungeon. He was unable to accept the party invite, so MewMew stayed outside to try and invite him. A bug occurred where wave was unable to join, and a further bug occurred when Mew finally joined the dungeon.
(At this point I derped and deleted the snip of the bug).
Basically it showed Mew as being inside the dungeon, whitenamed, and he didn't show up on the party list as being in the map (it showed him on Ice Caves 2F) . We killed tiger and he still obtained a chest. Wave was unable to join the dungeon throughout.