I heartily agree, sitting in should be something one is invited to do.
That said, I must confess to one serious violation in my past. When I got married in 2001, a buddy of mine held the guitar chair in JGB, so we wound up with Melvin Seals and the guys (and girl singer) as our wedding band. I admit that I volunteered myself to sit in with them. However, I was the groom and I was paying them.
When it came time for my showcase, I was admittedly a little nervous and intimidated. I looked over at Melvin, and he was giving me the look. You know the look. Heck, I've given it myself. He had no idea if I could play or sing, but there I was sitting in due to the circumstances. I played the opening riff for The Weight, sang a few lines, then looked over at Melvin again, who was looking right back at me with a huge smile. I immediately relaxed and enjoyed my time with JGB. One high point of the wedding, at least for me, was when my lovely wife went over to Melvin during a break to thank him, and he said to her: Your old man can really play.
But absent an unusual situation like the above, sitting in should be by invitation only.
Bill, tgo