The general rule for yichud is that a man and a woman who is not his wife or a woman and man who is not her husband may not be secluded together. If another adult can enter the room at any time without knocking, there is no problem with yichud, even if the adult is not present initially. Yichud does not apply whenever:

• A woman has a lockable door that only she controls, which is locked (in this case, men are permitted elsewhere in the building).
• Direct descendants or ancestors are in a room together (in this case, all other men/women combinations are permitted--even if the other people are not related).
• Three (or more) women and one man (or more) are in a room, except when they will be sleeping. At those times, four (or more) women and one man (or more) are permitted in a room or enclosed area.
• Two (or more) men and one (or more) woman/women are in a room.
• Other people have keys to the room and may enter at any time.
• One (married) woman is with one or more men and the woman's husband is in the same area (RMH looks at the local business district as the “area,” so wherever businesses would be advertising or marketing would be a local area).
• For other cases, consult a rabbi.