Just out of curiosity, I wonder why the MC is still there? Given that this is a gag manga, it's not like it has to be rational, but just wondering why she didn't just say "Ah, I'm in the wrong place! Sumimasen."
@ dreamingsamurai if she did it would make the group unbalanced and awkward so to preserve the harmony of the group she was planning on staying til the end and leaving without further confrontation.
since with gokons its common for on group to turn up first so she wouldnt have realised there were no guys at first glance