With Rumia present, more like she's settled in with Team 9. Either way, it means trolling people like Alice, with Halloween being an extra-special time for pranking, so it's easy to see the appeal to her of hanging out with that group.
Let's see...Daiyousei's a mummy (sort of), Cirno's a jack-o'-lantern (sort of), Clownpiece is...Clownpiece, and I'm not sure what Rumia's green outfit is supposed to be.