New to multiplayer steam games

I heard of groups and such but im not entirely sure on how it all works.
Is there like a er… guild thingy? clan? that i can join that has players looking to play? Again i know nothing of steam since i usually play with just my girlfriend or solo.

Friends can invite you to start a mission and you can invite your friends, from the Steam Friends.
Groups are all custom, they can for example schedule Lore challenge runs for Battleborn where people in the group focus on helping each other complete Lore challenges.