When the mission ends, the system computes the maximal possible teamwork points (according to the time taken to do mission) and compare this value to each player's teamwork points. Any player who has less than 50% of the maximal teamwork points value doesn't pass the mission and get warped back to mission outpost. All other players move on as usual. Simple isn't ?
Note : After testing, the values might be adjusted for best possible results. i.e. the 5 seconds and the 50%.
Funny side-effect : players running missions for lazy teams would not pass the mission and get rewards

Now, what is the thing I didn't see that make this suggestion totally worthless ? You tell me !
So... /signed or /notsigned ?