The current roster system is a pain.
I'd much rather be able to do this:
1) Accept everyone you want to have the potential to join
2) First-come-first-served at the server level. If there are 30 slots then the first 30 people are in.
3) All others can spectate. If slots open up, it's again first-come-first-served to join
So you CAN micro-manage your roster but you don't HAVE to.
Additionally, with this system, rosters could "semi-lock" -- after the start of the fight you can't remove people but you can add new ones.