123.300 and 123.500 MHz are recommended by FCC.
The frequencies 121.950, 123.300 and 123.500 MHz are available for assignment to aviation support stations used for pilot training, coordination of lighter-than-air aircraft operations, or coordination of soaring or free ballooning activities.
Applicants for 121.950 MHz must coordinate their proposal with the appropriate FAA Regional Spectrum Management Office. The application must specify the FAA Region notified and the date notified. Applicants for aviation support land stations may request frequency(ies) based upon their eligibility although the Commission reserves the right to specify the frequency of assignment.
Aviation support mobile stations will be assigned 123.300 and 123.500 MHz. However, aviation support mobile stations must operate only on a noninterference basis to communications between aircraft and aviation support land stations.
See your related question for soaring example of use. From this other answer, the FAA position for air-to-air communication with aircraft other than gliders and balloons:
The air-to-air frequency is 122.75. Remember that everyone in the air is using this frequency; transmissions should be kept brief. A balloon pilot trying to contact a circling airplane would try 122.75 first.
The air-to-air frequency is the common one (122.75 MHz), but extended air-to-air communications for gliders and balloons and coordination with the ground support will be done on 123.300 and 123.500 MHz.
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