Here's how they work - I think
I need to get Al in on this one, since he knows how they work.
Here's what I know about 'em.
There is a small speaker that mounts inside the helmet. Al has his mounted in a temp fashion, with the wiring running through the padding in the helmet and the receiver mounted with tape on the outside. You can still wear ear plugs or an MP3 player without interference from the speakers. Al said that even at speed, it was very easy to hear conversation from the other riders. I tried it out and it was very clear and easy to hear. It sends and receives over quite a distance too - easily enough to work with the average riding group.
The radio itself mounts on the handlebar, or that's how he had his fixed. To send, you simply press a button and speak. If your radio is on, you'll hear all of the 'sends' from the other riders. You can have as many riders in on the group as you wish - they simply choose a common frequency on the radio to use. I suppose that if there was a lot of riders, that it would take some time (and common courtesy) to keep from just hearing a babble of voices. What I like about the setup is the low cost and the simplicity. It has a rechargeable battery that he figures lasts for about 8 hours between charges, but it also accepts AA batteries for backup.
It's about as techie as I want to get...