When you see active players send them a friend request. Then next season you will likely be in a league with them. I have around 60 friends now, all of them complete strangers in reality. My league is completely made up of top11 friends, and is very active.

An excellent way to find active players it to just look at who is bidding in auctions. But also explore other leagues. Look at the leagues of other teams in your cup and CL, the top teams are often active.