When we were kids, our neighbors and very good friends had a pool. Our parents were great friends and exchanged babysitting us kids on opposite days of the week so we swam in their pool often and took care of it when they were out of town.
Of course, every kid in the neighborhood wanted to swim but many of them were not friends with our neighbors kids, nor were they friends with the parents or knew them that well.
Their solution was that if the parents of the kids asked and they met them, they would allow their kids to swim in their pool, BUT that parent had to stay and watch their child(ren) swim in the pool. The owners feeling was that they had other things to do with their time than babysit and watch every kid in the sub swim in their pool -- and especially those who they weren't friends with. Now that I'm an adult, even though we don't have a pool, I understand with and agree with their logic.
However, in this day in age, THERE IS NO WAY I'd let kids swim in our pool (if we had one) Who I didn't know or didn't know their parents well. There is FAR TOO MUCH LIABILITY if someone gets hurt or something and people are far to happy to try to lay blame on someone else for an accident and willing to pursue legal matters to sue you. I personally would not take on that risk. Ask yourself this: do you want to risk losing your home and everything else because this neighbor girl brings a friend that you don't know very well to swim in your pool, thus making you responsible for her safety while swimming at your house? If anything happens, you are responsible. Drowning in like the #2 cause of death in children ages 1-19 yrs old -- it was just on the news again last week.
I'd tell them sorry, but if they want to swim, they have to bring one of their parents with them and their parent has to watch them swim. In addition, If I didn't know that parent very well, I'd probably have them sign something, like a waiver of liability or something. Sounds extreme, but I've met some parents who would happily say they'd watch their kids and then doze off in the lawn chair and not pay attention to what the kids were even doing.
Just my 2 cents.