How do you people afford parties?

I'm in my mid 20s. I have 2 friends with birthdays between Saturday and last night. I skipped the party last night because it was a club and I don't like clubs/parties and I didn't want to spend money out on a Thursday night. I had no idea of what the plans were for Saturday until someone posted on the private invite page for the event on Facebook saying, "Can't wait for Saturday!" Now, I kind of feel like I have to go and I don't know how I'm going to afford it. I spent money on food over the weekend (I was behaving myself until that point), and now they want me to go to a concert on Saturday. I don't even know the band. Is it normal for your friends to expect you to spend money on the group's adventures when you don't have that kind of money?