Your girlfriend is not on the crew love wave. While she manages to muster up quick hellos and pleasantries to your posse when they come around, she's only enthused about chillin' with them when her favorite joins in.
She will put on that "stank face" for every other member of your crew and complain about the mess they leave in your apartment. But the one she's got her eye on comes around, and she is smiling from ear to ear while offering him a drink, some food, and anything else he wants.
If she's giving one friend special treatment above others, don't jump to conclusions, but do be aware of their interactions going forward.