I have played a few matches when one team goes one person less than the other side (or more). While it may be due to disconnects or quits, there does not seem to be a way to bring extra players into the match.
I understand the game allows disconnects or quits to rejoin, but at the same time, one of the following has generally occurred with people in these two groups: they join but don’t participate, or don’t rejoin at all. Please assist?