Russ,
The wireless frequencies are a bugger to harness. Our school bought 16 mics 2 years ago, and before we made the purchase, my tech researched what other cyber activity this area used. He found out the police, fire and major cell providers and other potential threats to signal clarity before selecting the channels used for our setup. The simple rules are these:
1) all mics need to be on a different channel if you want independent control at the desk (mixer)
2) it is advisable that mics have alternate channels available in case of interference (ours have 10 selectable frequencies)
3) mics running parallel in the same channels should not cancel each other out, but there will be no easy way to change volume between performers. If you need a simple, easy way to get 4-8 mics in a single channel ...
We also bought this unit .... for quick setups, and emergency additional mics. Seems like a good bet in your situation: http://www.amazon.com/gp/product/B005BSOW32/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

Here's the part number in case the link fails:
Pyle-Pro Rack Mount 8 Channel Wireless Microphone System with 4 Lavalier/Headsets and 4 Handheld Mics (PDWM8700)

It got us out of a jam when my mixer was maxed out and I had to mic up 4 more kids in the cast. Good luck!