And I'm confused about his complaint.
A game yesterday:
Our party on Exile gets control of the Banshee, Gauss Hog and Scorpion in Infinity Slayer BTB. We're wreaking things, start pulling far ahead. The other team starts quitting. Soon, just one guy is left. He's doing his best, hanging in there, but it's going to be another nine minutes of misery or so for him, and a slow slog of hunting him down for us.
Then he gets a kill. A moment, later, another. And we're like, hey, he's pretty good! But he seems to be everywhere at once. So we check the score board.
And the other team is fully populated now, it's back to 8v8 from 8v1. Join in progress meant both the lone dude on the other team and our party finished up a fun game, and it happened without our realizing it was even happening.