One thought I had as a possibility as to why a guy would do this is that maybe if your relationship is new, he might not want to bring you around just yet because to some guys, introducing a girl to his friends means a big deal, like introducing to his family. However, then he should not ask you over for the weekend.
Also, if your family and friends knows his family and friends, he may take you for granted like you’re just a hangout friend (hangout friend with benefits?). He may not respect you like he would a stranger that he is romantic with because you’re just one of the crowd. Now if you are just one of the crowd, then why not invite you to the party?
It could be because he’s young and stupid, and does not know any better. He may not know that women would like certain treatment, would like to be respected, would like to be included, would not like to be expected to just hang around the house and wait home for him to come and fuck.
Believe me, when i was young, i tolerated a whole lot more in relationships then I would now. I had a lot of friends and at that time we did not call them friends with benefits, but it was all casual and I did not mind a lot of stuff from guys, especially if i liked them, hanging out at their place might be a better alternative then not seeing them at all. Now I know better.
Hopefully the guy has some spare money and can take you out places. I say hopefully because you sound young, and i know sometimes when you’re young there’s not a whole lot of spare money for things like restaurants.
So let us know an update (I am all about updates on Fluther and I will usually request one). If you would like, let us know the outcome, please. Did you go to the party?