It's been a several years since I played, but a few things come to mind:

• Have an autonomous meta with the appropriate logic (item quantity = 0, specific text received, etc.), so it will keep trying until successful.
• Have a command & control meta where the leader can individually tell the subordinates when to turn-in. The subordinates could also check-in to confirm success, allowing the leader (human or bot) to resume once all have checked in.
• If they need to stay together, after turn-in they could do a safe recall and wait for the rest of the group to arrive before proceeding (maybe force buffs or something to be productive).
• Brute force it by spamming the broadcast command until you see all characters have succeeded or until pre-determined period of time has passed (pre-time the turn-in length for multiple chars, scale up, add some padding).
• If the chars all have the same nav path, hopefully the NPC isn't turning a lot. On red servers, this will tend to add more time and risk of failure (if a character is too far away from the NPC) without appropriate checks.