This is known as an Aggregate Audio Device on OS X. combine multiple inputs as one virtual microphone. On the cheap? Use a virtual audio device. Sounds like you probably have everything you need to get started except a mixer or the better option your own board, which every venu should have sound equipment but so should most musicians and bands for their own home studio.

Most of the control knobs and faders are arranged on the top panel, so it is easy to understand the signal flow and what state the audio is in. Analog mixers adjust the volume and tone of input audio signals. They all connect and you'll get the best sound from such a setup. Mixers are broadly classified into three types, based on their internal structure. Basically how I'd do it: MacMini >Firewire> Pre-Sonus Board > WiFi > iPad. 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. Mixer, 2-channel 6 microphone/line inputs, plus 3 music source inputs 100 V, telephone and a call station input with priority & VOX 2 zones and 2-channel.

2 input audio mixer