rufioh is in the group and aranea has a big fat crush on him
meulin is in the group because she loves roleplaying (she keeps trying to ship their characters) (no one really cares that much) (aranea and meulin have very detailed conversations about the relationships among everyone's characters)
cronus joined at the beginning, but got bored with it (mostly with the fakey magic stuff) (plus he realized that just being in the same table top gaming group wouldn't get him into other people's pants)
vriska was there in the beginning, but didn't last long because she never got to be dungeon master (she always rigged it in favor of her character) (plus no one liked her because her character was overpowered and vriska wouldn't change her no matter how many times the other people told her to)
Tavros ends up tagging along with his older brother for a while and becomes a regular member after vriska quits (everyone likes him because he's in it for the fun and isn't competitive or anything)