You probably want something such as Motorola聮s 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.
Motorola T9500XLR Talkabout Radios

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 helmet聴or over a helmet, for that matter. You聮re probably better off with a speaker/microphone ($40) that clips to a lapel or jersey. It needs to be activated with a button, but that聮s easily done even when riding. Voice-activated wouldn聮t 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 they聮re line-of-sight devices and have a hard time bouncing signals around corners or over hills. Besides, you don聮t really want to have long conversations; you just want to track people down who are out of sight, correct? In that case, you聮re probably better off using wireless phones. Here in Port Townsend, where I live, that聮s what we do. If we聮re 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.
You聮ve seen our picks for 2007 Gear of the Year, and now the entire
国产吃瓜黑料 Summer Buyer聮s Guide is online. Check out this year聮s more than 400 must-have gear items.