Couple things Eileen.
1. I'm pretty sure we've hashed out that not everyone agreed this was a "Girls" night. I think it was called that at some point, but clearly there were guys there too, so, it obviously ended up NOT being just a "Girls" night, so, what were the guys doing there? I don't have a link, but didn't PP invite EA? I dunno, but it was a gaggle of friends, loosly associated through Allen high school, presumably with PP as the nucleaus, since it was her apartment. That's my take on it. It's not weird or mysterious or suspect to me that guys were there. There is no answer to "what were the guys doing there?" Because I think the idea that it was a "Girls'" night was over represented and misunderstood in the first place.
2. Why was the interaction between SB & EA sexual? They are sitting on the couch together. He's probably inching closer, getting hopeful, thinking of where this could go. (You asked where other people are, like SN & CM: it's an apartment they could be in the kitchen leaning against the counters, there might be other chairs in the living room. They are lounging around, chatting.) so it's getting late, SB tries to stretch out on the couch so she can sleep, but, because EA is thinking sex, he doesn't want to move. Maybe he encourages her to put her head or feet in his lap instead of him getting up and moving. I can imagine him rubbing her feet
rubbing her shoulders as she tries to stretch out on the couch. But she wants to sleep not have sex, or make out. He's not getting the nonverbal signals she's sending saying "no" she pulls her feet away. She asks him to move so she can sleep. She finally realizes, the guy isn't going to easily back off, so she gets up, goes down the hall to the bedroom to sleep. She stretches out on a bed (is it PP's? Does she have a guest room too? I dunno, but SB is tired) however, just as soon as she gets settled and comfy, the boy with wandering hands comes along. He comes in, he sits on the bed, maybe even lays down beside her, but she again gives him the brush off
or, maybe, given the dialogue we've been told, if PP is in the bed too. So the two girls are both in the bed together. he now realizes that she's not alone, so that's going to give EA & SB any privacy to make-out or have sex. So, he says, "If you're not going to sleep on the couch [out here in the living room, where we can be alone together] I'm going home"
About that same time, CM comes out of the kitchen and says she is ready to leave (she's finally found those darn lost keys) and EA, jumps at the chance to walk her to her car. We believe that she called SN to tell him she was still okay to drive as she approached the garage, but we don't know anything about where SN was when he received those calls
maybe he lives at SAL too, maybe he was parked somewhere different and was walking to his own car. Maybe he took the couch that was now empty when SB abandoned it.
Anyway
all those details are the product of my vivid imagination, but I've written them in to help illustrate how the couch interaction was "romantic" if you read between the lines. Clearly this stuff is seriously just MOO and full of speculation. Just ONE version of possible events.