I have played a house of 6 sims and the only time I get gigs for the two in the home who are band members (the other two are good friends and not a part of the active family) is when I have one of them selected and I'm playing them. It seems as if you have to keep your eye out and your head band leader has to be the active one. I have noticed in my short time playing that gigs don't come very often. Most of the time you have to actually click the stage and get your active band members to start the concert. I ususally do this early so they get there on time and that way they are all nearby to start preforming. NOTE: If you have Ambitions and Late Night it is supposed to automatically send all the band members to the gig, but sometimes you have to still prod them yourself. Then you have to click on the venue or usually the stage, to 'call other band memebers to the gig. When you get the option to preform a gig you need to start very early and get your active sim band members to get to the venue or place where the gig is supposed to be. (you also need enough space so they actually are together)

You get your main sim jamming and then invite the rest in join individually so they are all playing together. I sometimes have to tell them more than once to 'join jam session' once they are together since they often want to swim or play games or whatever. Make sure they are all part of the band and be sure to occasionally invite the band to your home to practice. I will assume you have a band together with some other sims? Once you have a band together you get the option to 'invite band members'. However I'm not sure what all you are doing in game so it's hard to say what is and is not working.

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