I know that griefing a team of randoms is bannable, but is it still bannable if you do it in a six man group? I remember seeing some players do this before but I'm still not sure if Blizzard has ever officially addressed it.

We are strongly opposed to this behavior and working on a policy to make it consistently actionable and detectable. Even if you are in a 6 person team, intentionally losing to lower your skill rating is not acceptable.

For now, feel free to use the reporting system and choose Griefing/Inactivity to report this (coming soon to console).