Griefing goes both ways. If we went with the pushing solution, someone can be an a-hole just as easily and push you into an enemy fire while you’re hiding in cover, or push you into a [censored because we haven’t revealed this feature yet], or mess with you when you’re trying to mantle up a wall. The griefing potential of my being able to define where you stand, with you not being able to do anything about it is just too great, IMO.
Well, they sort of will. I look at it this way: IRL, I can maneuver around someone in a tight space very easily by twisting my body as I move. Emulating this in the game would take a TON of work (like what Assassins Creed does when you walk) and not really be worth the compromise to other features, especially since we’re a first person game. But we emulate this basic ability of a human being’s natural movement by letting teammates walk through each other. Note you can’t walk through enemies… an enemy is a wall, and you’re best off smacking him down with one of your [censored because we haven’t revealed this feature yet] if you get that close 
However, there’s “sort of” collision with teammates as well. Specifically, there’s a slight repulsion that happens when someone tries to move through someone else that causes the mover to kind of do a slight shimmy. This repulsion also prevents players from literally standing inside each other (and thereby letting a little guy hide “inside” a big guy). Players are naturally moved away from each other, but it’s always the player who initiated the collision (so again, you can’t be shoved away from something you’re trying to do, by some dickwad, nor can some dickwad prevent you from going where you want to go).
Hope that makes sense. I have to say, in playtests from a first person perspective, it feels very natural and smooth. Hopefully you guys will like it.