We cannot set the frequency manually on the group management page (because yes, people do choose frequency on their own mostly in gamemaster milsim missions) and it is not taking into account the radio frequency used by the creator of the group.
Would be useful, especially if the radio reset to the group frequency on respawn ? Which it should.

Here the squad (BRAVO) frequency is 42Mhz, but i cannot set it for the group either on creation or editing of the group.