You must, please please please!!!! Make it so that force field is one way instead of no way. In fact that way you could leave it on all the time. Make it so you can get in but not get out. That concept works really really well in Global Agenda.
You dont know how much heart ache it is to yell at people to get inside before the force field goes up only to be standing there thunderstruck when they are left outside.
Honestly the only trouble I've had with it is people not paying attention.
That's solved by simply waiting, doing a head count, or asking if everyone is ready. A teleporter that opens a dialog forcing everyone to click 'ready' could work too.
If you want to over complicate things though Cryptic could try a air lock thing. With a outer door always open, and staying that way until is inside. Then a outer door closing, a inter door opening, and everyone being forced into the fight.
Honestly there nothing that wrong with the system as long as your willing to wait 3min or just talk to your team.