1. Pretend that AFKER doesn't exist.
2. If you notice the person is AFKing at the beginning of the game...just leave and join another game.
Simple solutions that will help on the long run
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But, that doesn't actually avoid the matchmaking.... you just leave and join another match made game? Subject doesn't match content here :P That said, if you really want to avoid matchmaking, then bring a couple guys in to the fireteam, and kick them. I imagine though, that it will be just like other strikes, where if someone leaves they plop in a replacement.