I had a friend who would happily go along with the round, but when it got towards her round, would drink very slowly, so she wouldn't go to the bar. I used to find it really embarrassing. I thought I was the only one who noticed, but a couple of other people have mentioned it to me since then. I think it would have been fine had it not happened so very many times, or she excused herself from the round at the outset, or just didn't say yes every time it was somebody else's round!
My friend also used to take the mick when it came to birthday presents. We were part of a large group, and when it was somebody's birthday, one of us would buy a present for the birthday girl from the rest of us. The idea being that each of the group would take a turn buying the present. She never took her turn, and would say things like "I think I bought the last one" or we would choose a present and get to the till and she wouldn't reach into her bag or anything.
We once went to a wedding, and she didn't take her purse! How she thought she would get home (we were getting taxis) or pay for drinks, I don't know.
I'm happy to say that she is a lot better now! I don't know if anyone said anything, but it doesn't happen anymore. I'm glad as she is a really nice person, and it wasn't showing her in a good light.
Now that I am older, I think I would say something along the lines of "Is it my round? No, it can't be, I've already bought one" and then leave a pause.