Ohhh, you found my old post in the biforum!?!
Yes, you have to put this sentence into the init line of the soldier, which you have spawend ingame with the RTE.
I used this in the "Battlecenter" maps so every player was able to control an AI unit for backup. Medic ...
The bad thing, i got this code from Sickboy back in 2010 and i think there is a part missing saying join or something like that but maybe sombody else can also help you and complete this code line.
If it helps, i created a post yesterday in the ArmA forum with a join group script maybe this helps you too. It is working now :D
If you ned more help let me know ... hope i can help a bit.