ok, basically you can use a "not present" or either a "present" trigger.
Once you grouped this trigger with your group (same way as you add another unit to a froup), there will be some new options in the trigger, as "Grp Leader present/not present", "Grp present" and some others.
The idea is, to set the trigger size that it covers the whole mission area, and use the "Grp not present" to fire the trigger. Don't care when your Group starts outside the trigger, the trigger wont fire as we put a SECOND condition in that has been done.
As Artak said, in the Condition box put
this AND meeting
"this" refers in this case to the settings of the trigger you defined above, and the "meeting" variable has to be true for the trigger to fire.
As you guessed right you define the "meeting" variable in your init.sqs as false.
Switchin it to "true" is pretty simple. I guess you make something happen when you meet your informant (cutscene, conversation). Just use "meeting true" somewhere in the stuff that runs there.
:edit:
i just saw your old topics were still active. Please, once your prob is solved, hit the "solved" button at bottom left....whenever you think there's still something you need to know about, you can unsolve it at any time.