Well balancing is part of beta testing - so that should be taken care of.....
If you look at the N:TW MP campaigns they have rules - some games autoresolve all battles others fight them. I know there is a 'battle drop in' feature in N:TW - I've never used it but if the other players wanted to stay involved they could use that feature. I'm not sure if there is any control over who drops in though.
I guess it would be limited to two players - unless there are multiple armies and then someone could be iin charge of just one army each.