My english is bad enough to missunderstand you, but I think you need to create a radio channel:
Place a trigger on your map and set its activation to "Alpha" (radio channel number 1). It means that it will be activated by the player, through the code 0-0-1. Then at the activation field, place the commands you wanna exec, like
player sideradio "String"
player join ObjNull (if the player wants to leave the team)
etc, etc.
A hope this could help you