When grabbing the commisoner what is Tyler Durden actually doing?

I always took that to mean one of the Project Mayhem members locked the door. Edward Norton's character imagined/remembered that it was him instead of someone else doing it. Presumably this was guided by Tyler.

We see Edward Norton's character imagine himself doing things at other points in the movie. For example during the Tyler and Marla having sex montage, Edward Norton walks up to Tyler's bedroom door and peeks in. Tyler and Edward Norton have a short exchange, Edward walks off, then Marla asks Tyler, "Who are you talking to?" which indicates that Tyler was in control of the body at that point.