I posted this in the childhood years but would love the input of preteen/teen parents!
Pardon me while I
As my children went to high school, their social circles expanded dramatically and there was no way to do the extensive "getting to know you" that we did when the kids were younger. At first, they went to sporting events and then out for pizza, and I would always offer to be one of the drivers so I could at least get to know the kids. When it came to parties, I was always happy to host. When they went to parties at another house, I had no problems calling the host parents to introduce myself and ask if they needed chips/soda etc. When I picked them up, I always went to the door and chatted with the parents then, too.
This is obviously so much easier when they were younger. When I did the drop off/pick up, I always had a chance to scope out the houses a little and chat with the other parents. Often they were familiar anyway, from school events or extra-curriculars.
Once they started navigating public transit systems on their own (about age 12), I had to rely more on their own good judgement. We did lots of work - discussion, role playing, exploring "what would you do if...." scenarios, They proved over and over again that I could trust them.
Now that they are teens (almost 18 and almost 15), I don't make an issue of it at all. I've met many of their friends when they come here for visits. I've met some of the parents but not all. Some parents still come to the door for pick-up and we'll chat. They all seem like nice people. I think my teens have developed very sound judgement about people. I don't think they could have, if I didn't allow them some practice with it when they were younger.
We still have the discussions. DS is planning some gap year traveling. A couple of days ago, when we were on a short weekend trip in a hotel, we ran through some "what would you do if...." scenarios.
We keep our home open to their friends. For a while, DS hosted practices for his garage bands in our basement. DD likes to have sleepovers. We give them free reign of the refrigerator and pantry and try to keep them stocked. We encourage them to invite their friends away with us when we go to our cottage - and I'll have at least a telephone conversation with the friends' parents before we go. I still try to get to as many school events as I can, although they are fewer than elementary school, and I usually meet a few other parents there.
I would say that by middle school, it's harder to get to know all the families your child might visit and by high school, even tougher. At least, that's the case if you are living in a typical urban environment. I haven't lived in a small town, so perhaps it is very different there. I think, however, that as they grow from middle school to high school age, they develop sound judgement and good decision-making skills that can help you feel more comfortable about letting them venture out without you scoping out the territory first.