It depends. Sometimes everyone transports their own kid. Sometimes I put my friend's booster next to DD's seat and take two. If there is a specific friend that often takes her for an afterschool playdate, then make specific arrangements with that person. Same as you would if Grandma picked her up. I've also found that friend carting is different than it once was. When they are smaller a lot of parents prefer to do the driving. Your arrangements may vary by the family. I typically drop my child off/drive along with and take bigger people if riders need to be shuffled around. I don't see it as an inconvenience to keep my child safe vs putting her in the wrong type of seat.
My uncle also used to roll around in the back window. They were very lucky not to be in an accident when he was doing that. I've seen accident footage from the 50s or 60s when they had things like huge steering wheels with spikes on them and no one used seatbelts. No, thank you.
Please be very careful with yard sale finds. If you aren't 100% sure that the seat hasn't been in an accident, you could be doing harm by using it.