I really don't care if a girl doesn't react well when I approach.
She's obviously a lesbian, so whatever.
Parties are completely different than approaching girls in public, and the biggest difference is exactly what you're having problems with:
They go back with their friends, or talk to someone else.
In a public setting, you can just approach, talk for a bit, get her number, and leave because you both have other shlt to do.
For my personality and lifestyle that way always works the best for a number of reasons.
An important thing to remember about parties is that you shouldn't talk to the same girl for too long.
If there's a spark, great. Get a little fire going.
Then go talk to some other girl. Then go back to your "target".
Your target will now see you as more valuable because she saw you talking to that other slvt and wants to have you to herself.
You should be escalating physically at parties too. That's how you speed the process up.
Girls know how the game works, and she expects you to come chasing when she runs back to her friends. Fvck that.
Show her how little you care about her, and suddenly you are a lot more interesting.