Place the following barrier setup on all bosses in every duty that requires a party. While you're at it, why not add one to the start of the duty as well! Works as follows:
1. Barrier A is down and barrier B is up.
2. All party members must pass barrier A and stand between barrier A and barrier B.
3. Barrier A is raised and barrier B is lowered allowing you to take on the boss together forcing impatient players to wait for the party.