Welcome to the forums, guys
When a player activates a custom action, arguements are passed to the script. The player who called the action will be:
_this select 1
So if you use the line:
_unit = _this select 1
in your script, then make your chopper go to _unit.
However, I would definately put a piece of code at the start of the script that removes everyone's action to call the chopper... otherwise you may get more than one person calling it... and as a result get conflicting movement commands