You probably want something such as Motorolas T9500XLR Talkabout radios ($80/pair; motorola.com). These use both the common, short-range FRS (Family Radio Service) frequencies, plus the longer-range GMRS (General Mobile Radio Service) frequencies. The former have a max range of two miles (practical range is much shorter), the latter a max of 25 miles (ditto). Note that you MUST have a GMRS license from the FCC to use those frequencies. The cost is $80 for a five-year license.
The radios are small enough to fit easily into the pocket of a bike jersey or a Camelback hydration pack. Motorola makes a headset with a swivel boom ($15), but I doubt it would fit under a helmetor over a helmet, for that matter. Youre probably better off with a speaker/microphone ($40) that clips to a lapel or jersey. It needs to be activated with a button, but thats easily done even when riding. Voice-activated wouldnt work because the sounds of the trip, and even maybe your breathing, would be apt to set it off.
My real answer, though, is to not bother. In hilly terrain the radios will be virtually useless because theyre line-of-sight devices and have a hard time bouncing signals around corners or over hills. Besides, you dont really want to have long conversations; you just want to track people down who are out of sight, correct? In that case, youre probably better off using wireless phones. Here in Port Townsend, where I live, thats what we do. If were at our meeting point and trying to track down Clark or Dave or Jason, out come the wireless phones. Wireless phones work in far more areas, have better range, and you probably already have them. Plus, in the event of an emergency, you can simply dial 911.