Monk i bet every instinct is telling you to go to the party tomorrow. WRONG
You go out with the boys. You send her a text saying something like "Im going out with the lads tomorrow, ill see how i feel on Sunday might give you a call". Then you go get sh*tfaced, dont text her dont call her. Its hard but it must be done.
Then on Sunday you send her a sexy and straight to the point text, and leave no doubt in her mind that you want to hit it or cya later.
Your over thinking everything. Do whatever the f**k you like. She doesn't like it then so be it
UPDATE: okay so she sent me a fb message saying 'we should hang soon, my friend is having a party on Saturday night, I'll try and get her to let you come?'
While this is all well and good, I'm going to town with all my good mates on Saturday night, my plan was to get epically sloshed and have a crazy night with all my best mates, I don't want to choose her over them, but at the same time, I want to see her.
Advice pl0x.
don't know about wisdom but girls like to play games. My opinion is you should never buy into their games (very bloody hard) lollisten to this wisdom
You say "I'd love to but I already made plans with my mates. Why don't we do Sunday instead, or do you and your mates wanna join us after the party or something?" Or something to that effect. Don't just blow her off, she'll get pissed, but don't drop your plans for her either.