You have to set up 1 radio trigger for each player. Then, in your init, set the radio commands that don't correspond to the player to "NULL".
Example: Your men are named driver1, driver2, driver3, etc. Your cars are named car1, car2, car3, etc.
Radio Alpha
car1 setPos getPos driver1
Radio Bravo
car2 setPos getPos driver2
Radio Bravo
car3 setPos getPos driver3
etc. etc.
;; begin init.sqs snippet
?!(player == driver1): 1 setRadioMsg "NULL"
?!(player == driver2): 2 setRadioMsg "NULL"
?!(player == driver3): 3 setRadioMsg "NULL"
?!(player == driver4): 4 setRadioMsg "NULL"
?!(player == driver5): 5 setRadioMsg "NULL"
?!(player == driver6): 6 setRadioMsg "NULL"
?!(player == driver7): 7 setRadioMsg "NULL"
?!(player == driver8): 8 setRadioMsg "NULL"
Now, the driver only sees the radio command to return his car to him. So driver1 would press 0-0-1, driver2 would press 0-0-2, etc.
It has to be this way because the activation of a radio command is public, and there's no way to determine who triggered the radio. So unless you want everyone's car to return when 1 person triggers it, you have to separate them