Saying "is there room for one more?" is good. That's what I would say in that situation. I'm from the U.S. and that's what most people say in the U.S. In other English speaking countries people may say something different.

My only point of English in your answers. Concerning “I’m not sure I’ll fit in there” I wouldn’t say “in there”. I’m not sure I’ll fit. (Talking to someone outside) Will I fit? (Talking to those inside, said hopefully looking for a “yes” answer) If the elevator door is open it doesn’t seem like an “in there” situation to my ear. I agree with your and others’ “Is there room for one more?” Ways to ask if there’s enough room If there are a number of people getting into an elevator and a person isn’t sure that (if) he’ll fit, what might that person (he) ask or say?

Is there room for one more as Lenice said is probably the most common. There are lots of ways you could express the same thing though. Can one more squeeze in? Will I fit in? Can you make room for another?, etc. Humor could work also such as alright everyone, squeeze together. People would think that was weird but it might work.

Your question is really good so the only things I can find are minor. Your first sentence is too long and should probably be split. An easy way is change the first If to Suppose then end the first sentence at situation. Last P.S. are both capital letters. I did say they were minor.