This doesn't have to entail just girls.
This is the act of two or more friends spending the night at one of the persons house. Usually involves the visitors bringing sleeping bags and sleeping on the floor. Common activites tend to revolve around a sleep over: Pillow fights, Board Games, Dungeons & Dragons, Makeovers, and talking about the opposite sex.
Most Pajama Parties involve a sleep over
Hey, let's call up everybody and have a sleep over this weekend.
sleep over when girls get together to talk and comb each others hair
this sleep over is a total freaking drag