Session work, in a way.
Started when I was playing at a youth group at a church (didn't care for the beliefs, but enjoyed playing with people, and was friends with a bunch of people there, and the original youth pastor was really cool and made it lots of fun without getting too preachy), as I'd basically learn the songs an hour before playing them, which made it fun to try and add my own things mid-song, the 3rd time ever playing them
Similar experiences in college, playing in the studio for fellow students, and playing in a friend's band live.