I guess you're not from Zagreb?
Here there are large group of players from every team. I met over 20 people just by playing PoGo and I spend quite some time with them (taking down gym etc.)
Mostly there are Facebook groups from teams plus the whatsapp conversations from little groups (or not so little).
But I pretty much know everyone who is actively playing (maybe not everyone in person, like spoofers and players from the other part of the town).
If you want to meet someone, and I recommend that (so much more fun to play with someone), try facebook groups.