Overwatch, CSGO and virtually all other team based FPS allow you to queue solo, as group, or a full 5 person team. This is outside of a specific league. There are dedicated LFG sites for different games to help find groups ahead of times. Generally you will be matched against a similar team, and different games use some form of skill based matchmaking, but depending on how many players there are, what modes, what region you are in, as a solo player you could be matched against a premade or vice versa.