Your band has a loyal hundred people following. Another local band also has a hundred people following. A smaller third band draws about 25-50 people. Why not combine all three? Now, the normal audience you’ve drawn has more than doubled and you are possibly filling a 250 person venue assuming minimal crossover.
That’s not half bad. Be careful though, use this creative tactic irregularly as you do not want people becoming used to the idea of your new trio always performing together. But, if you quickly need to up the capacity, this isn’t a bad idea if the venue permits it.