I'm 22 I dated my ex when I was 17 or 18, we've talked on and off since then but nothing serious just as friends. But the thing is over the last few months I've grown to like one of her girlfriends she happens to be close to, what should I do?

It has been many years since you two have dated. Since you are talking as friends, tell her you like one of her friends and tell her you hope it's OK with her.
Does her friend know you like her.

She will have to understand because after all you are just friends now. If u like this girl, go for it. U aren't in a relationship with the girl u dated years ago even though u still hang with her