Read the rest: 3 random players, and 5 known-goods. I'm sure someone can say how they're assigned in a better way, but all that's lost on JtD, apparently, and it's only a little ^airquotes random ^airquotes unless you let it be otherwise in some way. Airquotes because the only random is the pool the matchmaker draws from, which isn't really random to begin with, is filtered through said players' skills, and only accounts for 37.5% max anyway. There isn't even a bad choice to draw from, once you've cornered the odds with a platoon, really. The worst you'll get is 3 players that are inconsequential to the match, because 5v8 is very winnable if those 5 are in it to win it, communicate, focus-fire, and work as a team.

This is all just stacking viability enhancements put there for this very reason. Use them. When you don't, it's like leaving your front door open for some random-ass to waltz in and gank your shizzle. Hard to keep face complaining later about it, IMO. Just take the steps you can to keep your shizzle unganked.