2-way radios are cheap and effective.   They need 2 AAA batteries to run and they last for around 30 to 40 hours.  The 2-way radios are able to fit into your pocket or clip on your belt.  The 2-ways have 14 channels, the ones that we are going to be using do anyway.  There could be 14 total groups and they would only be able to talk to people in their groups.