Recently, I have built a private portal system for myself to go to and from Spawnville, and found that LeoVII came through the nether-side portal as I was finishing securing it. After a little chat and fighting off some Ghasts, I guess he also has a portal near spawn, that happens to link to my portal in the nether. If LeoVII had gone through my portal in the nether, he would have wound up in another secured portal-room of mine. For the time being, I have added him to the list of people who can use my portal system.

If you find that you are inside a secured portal-room, then type /cinfo into the chat and punch the door and/or a furnace. This will tell you who probably owns that portal. Use /tell [name] to contact that person, to ask them to let you out. It is likely that you are trapped in their portal system.

if you are in a hurry to get somewhere, you can try breaking a block or two to get out. (be sure to place the blocks next to the portal) If you cannot find any blocks that are breakable, then try logging in and out in as fast as possible. This should place you in the nearest open space.

Of course, ask in the chat for someone to get you out. If an op is feeling particularly kind, he/she may tp you out of the portal. If you know who's portal you are trapped inside of, then ask them to let you out.