First band I ever played in, I didn't even know the difference between a major and a minor scale? We played covers, and I just learned the chords, and the guitar solo by listening to the records. It wasn't until I met a guy in the local music shop who was looking for a few gigs, and I invited him along to jam with us that I learned any music theory at all! He didn't read music, but he taught me quite a lot in the way of scales and chords that had 7ths and 3rds, and timings. And how to write out a chord sheet to follow a song. Here's one I found a few days ago!
Which was pretty easy to read on stage!