I was first going to offer some general suggestions, but then I thought "hell, I think better with the editor under my fingertips" : so I went in and it took me about two minutes to figure out a foolproof way :
Trigger : A/B (whatever you want)
Grouped (F2) to the group in question.
Activated By : Whole Group - Not Present
On Act: hint "one out!"; {_x setcaptive false} forEach units YourGroupName; {_x setcaptive true} foreach thislist
And that's it. AFAIK if you make it repeatedly, it'll keep doing it for as long as there're units inside the trigger. I'm sure you can figure out how to make it for instance re-setcaptivate people who re-enter the safe-zone and so forth, if needed.
Note : it takes a number of seconds for OPFOR troops to actually realise you're no longer setcaptive, so if you want them to act faster at someone leaving I suggest you add some kind of Reveal function (for instance a larger trigger surrounding the smaller one, which reveals everyone entering it to the OPFOR troops).
Have fun!
Wolfrug out.