Before I start I want to stress that I'd like very much Fizzer's opinion on this in terms of being easy/hard to implement.
So, when we create a round-robin or knockout team tournament we can let other people forward invites. If that's the case many people that we (and the host) do not know will likely join. When we create open games we can invite other people and depending on their response we can always switch them with an open seat. My question is, wou;d it be hard to implement similar invitation system to the tournaments? For example I join empty team A, I forward invites to 50-100 people, but 2 of them I mark as "team invite". They would appear as my teammates with waiting for confirmation which would enable many good teams to form. As it is right now a lot of time and effort is required to gather a reliable team, since random people might join. Of course that would not stall the tournament, since host could at any point un-invite those people if they are not responding for a certain period of time.