When our kids were little, we knew all the friends and parents. Now the kids are in a catholic high school. No more local town kids. Friends are now from all over the state. You barely meet the new friends much less the parents.
Just need to hope you've taught your kids well. Our daughter is good friends with a new girl we've met a few times. Met parents once, seemed nice, smart, articulate. Mother is a doctor, father was in tech and sold his business for multi-millions a couple of years ago.
Just found out from my daughter, who just found out, the father has been in drug rehab a couple of times. Supposedly clean now but who knows.
If my daughter didn't tell us, we'd never have known and would have let her go skiing with the friend, brother and dad over the holidays. When she asked us if she could go and said the father was taking them, mother couldn't go, we told her no. She was disappointed but even confessed she was a bit nervous going with him.
You can't protect them forever. Just have to hope you've taught them well enough to make smart decisions.