I'm going to let my girlfriend choose whether she's going to watch the endgame with me or with her friends
Why are you concerned with who she watches the movie with?
Well, because I paid for the tickets and I don't want to feel like I was conned
It is appropriate to allow your partner to make her own decisions. You should have a good mix of together and friends time.
I was only trying to have fun with her. I feel like she lets her friends spend more time with her than with me.
Your partner has some friends she needs to spend time with too. What many couples do is when one person spends time with their friends, the other person will also spend that same night with their own group of friends. That way each person still gets to spend time alone with their friends!