Can being in a room where others are smoking pot cause you to breathe it in and have a small amount in you?

Smoking pot. Yes. This is similar to second hand cigarette smoke that effects everyone in the home where smokers smoke.
Debatable. Second hand smoke is very unlikely to cause significant blood levels. Some have even looked at this in a closed environment like a car. Most urine tests have cut off levels that could not be likely reached by second hand smoke. It is unlikely this argument would work in any legal proceeding.