Players connot join the party while you are engaged in combat

[Lerris]

They may be able to get around this by not booting the lost player for something like 10-20min. They could just have their name gray/red till they log back in.

This is how D&D Online handles it. In fact, if the entire party logs off and comes back, the party' actually remains intact even several days later, until the current party leader breaks it ( leader reverting to the first person to log on ).