When your playing with a bunch of people how do you bring back the guitarist or something while your a drumer or a singer?

Um, you activate overdrive.

-DM

For the drummer, you have to do one of those freestyle sections and hit the last note it gives you. For the singer, when you see a yellow design like thing between words, sing or say something into the mic, or tap it and you will save a bandmate that has failed. Of course, like the person above me said, you need to be in overdrive for this to work.