To use amateur radio, you must be licensed by the FCC. (How to get this license is a subject for another post.) There are three license classes: Technician, General, and Extra. The entry-level license, Technician, grants you all amateur radio privileges above 50 MHz. Since rally communications take place on the 144 MHz band, this is the only license you need for rally communications.
Update 2/21/2020: There is some serious talk about using 80-meter NVIS single sideband for the South Arm / Icicle Brook stage at the 2020 New England Forest Rally, which has always been problematic for 2-meter communication due to the hilly terrain. A General or Extra class license will be required for these communications. Operators for this stage will be hand-picked by the Chief of Communications out of qualified individuals. All other stages will operate on 2 meters as usual.