I think I've joined a band

I met with three dudes, one of whom I know pretty well on Wednesday to "jam". It was pretty funny. I've never been part of any sort of musical process, so I really had no idea what to expect. Would they want me to stand in front of them and sing Big Spender? Should I have written something? Where do I come in? How do I take part?

What actually happened was the four of us sat in a tiny room and they tootled around on instruments while I listened. Then they played Upside Down and I sang along to give them an idea of what I sing like. Which apparently was grand, because we went on to make up a song called We Make Party in which I sing in a German accent - a surefire international hit.