In order to protect a portal in the nether I have an obsidian room with an iron door, which works off a switch in the room. The problem is that if I leave the room and someone else on the server closes the door from the inside I can't get back to that portal.
Powering red stone on the outside of the door doesn't seem to work, as the switch inside is already powering it, so I'm guessing something like an XOR gate will be the solution, however I am finding it difficult to see how to make this work in practice.
Best Answer
I would suggest using buttons instead of switches. You push a button, the door opens, then the button deactivates, closing the door.
It's true that the delay is really short, maybe not even enough for you to get through. This problem can be solved using repeaters to build circuit delaying the closing signal. I currently don't have enough time to try and build a working solution, but if you won't figure it out on your own by then, I'll do my best to update this post later in the day.
Edit: Just for for the sake completeness: instead of buttons, you could also use pressure plates. Just place one in front of the door from both sides. Be wary though, because mobs can also trigger them. (I almost lost my mind once, when I had a door wired like this and a damn chicken kept stepping on the plate and banging with the door. I had to replace the plate with a button.)