Gryffinclaw Princess is quite right that there are a couple of cosplay groups in the Cosplay area, despite the fact that they're primarily chat/meetup threads for a handful of people. But it's not because that's how things should be organized, it's because they were there before the organization changed. They mostly behave themselves, so we haven't moved them.
If a thread is primarily about meetups, then it belongs in Plan Your Own Adventure.
If it's primarily about chat, then it belongs in Posting Games and Chat.
If it's primarily about a cosplay topic (i.e. wigs, sewing, cosplay for a specific anime), it belongs in Cosplay.
Unless the thread sharply changes focus, it is in the right place.