It depends on what you are doing, I think. If she enjoys you taking them to the mall to shop and see movies that is one thing. But if they are trying to take advantage of you in order to have your house be the "cool house" where they all go because they can do whatever they want then I say No Way!!
its cool to be cool with your daughter but you need to be her mother not her friend right now friends comes when she has her own kids right now you have to teach her what she needs to know to survive this crazy world
That depends on so many factors. In general it certainly doesn't hurt to spend time with your child & her friends. As long as she isn't using you for a "safety net" or avoids being with her friends alone, I would say enjoy it while you can.