Getting A Sims 3 Band Gig
Judging by the confusion in various online Sims-related forums, getting your musician Sims' band to play a gig is one of the more mysterious and difficult features of the Late Night expansion pack. If not ever!
First of course you have to form a band. To do this, control the Sim you want to be the lead player, and then click on someone else who has musical talent. You will have "Band… Ask to join band" as interaction options.
If you spend time in the various clubs that have musical instruments available, you will inevitably run into a Sim who plays with at least a minimal competence. Often your Sim will have a wish to "Ask so-and-so to join the band."
Once you have formed your band, your next step is to wait until you get an Opportunity.
You may have to wait for a really long time.
Most people report getting the "play a gig" opportunity within a few days of creating their band. I had to wait almost nine in-game days before my lead Sim got the opportunity. It showed up on his Opportunity panel as "Play for 2 hours at such-and-such venue." My opportunity did NOT show the day or time, so make a note of it when you first get the pop-up message.
The gig will usually be a few days away. This gives you a chance to improve your musical skills, get well-rested, and get really annoyed by the delay.
A few hours before the gig is supposed to start, the game gave me a pop-up notice that it was coming up. I think next time, starting when I get that pop-up notice, I will have my band members assemble in front of the venue before it was time to start. Playing a gig is basically a group outing, and you know what that's like - someone's always late, someone's car is always super-slow, someone always accidentally cancels the interaction and you have to invite them again. It's like herding cats sometimes, I swear.
Instead, I waited until it was time for the band to start, then I zoomed out to city view and clicked the gray pushpin star for the opportunity. This automatically put all my band-mates into the same group outing mode. I clicked to control the lead Sim, and had him go to the bar and start playing his instrument. (Click the instrument and choose Gig -> [music style].) The other Sims followed suit, and soon enough, they were all playing a proper gig.
(Until the final member took the stage and started playing, my lead Sim stood there tuning his guitar and warming up. It was cute - a nice touch on the part of the game's developers!)
As you are playing you can click on any of your band-mates and choose "Sweet move." This will make your Sim do a special little fancy bit, which pleases the audience.
Once you have played for long enough, you will get a notice from the game that you can collect your money. (I'm actually still not sure if I got paid or not. Next time I'll pay more attention.) After your first gig, you will also get "Garage Band" as a part-time occupation show up on your Careers panel.