taultunleashed

If there's too many mobs inside a team mission for your team to take on, just have everybody leave the mission area and disband the team. Enter the mission alone and then re-invite all the members of your team while inside(just /invite <name>). You will have less mobs inside the mission.

Also, this same trick can be done to respawn mobs inside a mission. If you have a really easy mission and just want to take out hordes of mobs with AE's for fast exp, you can have everybody including you leave the mission area, disband the team, then reinvite everyone and reenter.
I think you can also simply have everybody leave the mission area, change to another mission, then change back, and that may respawn mobs, but I havent tried that yet.

Does this work?

Ive had a hard time having people join missions inside an area once im alrady in without me leaving, and vice versa when entering the same area when not in a party.... Don't you have to enter as a party for it to give you the same isntance of the field/dungeon the missions in?

This works. I tried it out. Had a full group and entered a mission, then left and disbanded, entered the mission then invited them all. Mob were slightly lower level and a lot fewer.