What most pro and amateur teams have been using for the last few years are small radios on the small of their backs (either in a jersey pocket, or better yet in a pocket of your bibs/skinsuit to be held closer to the body). Single earpiece with inline PTT (push to talk) mic. Have tape (medical tape works well) handy to keep the earpiece in.
Riders can usually hear OK this way, but it's critical to speak slowly and clearly. You want to be clear on who is talking when, and why. The protocol is normally silence, director (in car) and team captain can speak when needed, other riders only speak when spoken to, or to provide critical information (crash, road hazard, etc).
Voice activated stuff doesn't usually work well. Either opens mics when it shouldn't, or you miss the first word. Besides, you don't want to hear everything a rider says.
I worked with a pro team at a stage race in May and they had some really nice, small, lightweight radios (about 1/4 the size/weight of a small Motorola talkabout), very thin. Didn't get a brand though, and not sure if they're commercially available or what they cost. Still the same earpiece & inline PTT mics.