Why would you each use a tablet? You can lock individual tablets to individual musicians. You can go with a mixer as mentioned above or you can use something a little more high tech like a Pre-Sonus / Studio Live sound board that'll do it all including your in-ear monitors with iPad control, each band member can have their own iPads or you can all share one.
As a theatre technician for a multitude of shows that involve everything from bands to 60 people musicals, you have a few options.
