Summary:
Issue a command to any group member and they'll do it before you say it.
Step by step:
1. Start the editor.
2. Place a player-controlled squad, and add the following to the player's init field:
{ _x setCombatMode "GREEN" } forEach units group this;
3. Place an enemy unit 10 meters in front of the player group.
4. Preview.
5. Select all units in the group and order them to "Fire at will".
6. Notice how they start firing before the leader is done saying the command.
7. Restart the preview.
8. Order all but one unit to open fire. Notice how they will wait until the leader is done saying the command. (How it should be).