Radio 2 Meter Ham or CB (Type indicated in each Run Announcement.) Most Run Leaders now specify that all participants on their runs have Ham Radios. Ham Radio is far superior to CB.
A license is not required to buy or monitor Ham frequencies and no license is required to use a radio in an emergency situation. You can buy a good hand held Ham Radio that will work great on the trail for less than $30 on Amazon or Ebay. Look for a Baofeng UV-5_