@Daisymay1000 ultimately, for one reason or another, it's best your relationship is over.
Truth is, I think there are lots of things going on here.
Firstly, you said there are 5 in the apartment - so her and 4 guys? If she is there for one of the guys, what makes you think it's yours? I can't see the other fellas being happy about one of them getting their end away in a 2 bed apartment.
Secondly, just because she is a 'pick me' (which it does sound like) doesn't mean that he WOULD pick her. However, the rest of your posts indicate some very deep rooted insecurity around this. That in and of itself is a problem whether it leads to you doubting your relationship and a lack of trust or whether it leads to elements of you being controlling - both are problems.
Thirdly, men and women can go on holidays together without it being more sinister. I have done it myself - one guy added on to our girls trip. Nothing sexual/romantic with any of us, just a nice guy going away with his friends. However, I can say that my OH at the time would have raised absolute cain (as he was exceptionally insecure, jealous and controlling) so I omitted that piece of information to save myself the headache.
Truth is, overall it sounds like you don't trust him. He's lied to you, which has confirmed for you that you shouldn't trust him. At that point, the why's and wherefore's become irrelevant