I would wait to tell them. I'm guessing you are in a dorm or something similar so these people are assigned to you? If that is the case, I would wait. There are people who will understand and be good friends, but there are a lot of people who won't. Better to wait and see what type of peopel you're dealing with.

I say just be friendly and communicate your concerns to them without the bipolar part. Like others have said, ask if you've upset them. Remember there are some unreasonable people who have their own issues (mentally ill or not,) and I hope for the best for you.