Oh Boy. We're playing an outdoor summer club thing. Big stage, big sound system, blah blah blah. The guy who sat in was really nice but the context was wrong. Young guys - here's the rule.
If the guys on stage are a lot better than you - do NOT ask to sit in.
This is the time you listen a learn. There are exceptions but if you don't know anyone on stage use this rule. I've been on both sides of this one and have made my share of mistakes. It's also important to understand that working bands often book tons of weddings from these outside events so every part of the show needs to be fairly professional. Please don't take it personally if someone says NO when you ask to sit in. That's what Tuesday night open mics are for.