OK, I tried calling another Sim to my band to make it 3 members but the option never appeared! Why is this so? I am sure that this Sim can play the instruments well as I have seen her jamming with me. What should I do now?
Make sure you're looking under the Band social when you click on a Sim. And for the record, pretty much any Sim can be asked to join a Band — their musical skill has nothing to do with it. However, if they're a celeb, you'll probably need to jump through the hoops and impress them before seeing the Band option.
Moreover, I don't like having 4 or 3 members in my band due to path finding issues in the game. If 4 is the official requirement...
It's not. Once a Sim has a single skill point in any instrument, the Band social becomes available. So, a Band could be two, three, or four Sims.