Depends on the original rental contract. If it states that other 'renters' will occupy the premis, you will need approval by the regie. Yeah, this is very broad and open ended, its to give the power to the regie dispite that you put the deposit down, your at risk of the whole rent, etc. but you need to re-read your contract de bail. But in the end, if you ask often they really make it difficult or refuse. So sometime just do it. Look im no lawyer or prividing any 'professional advise' .

Have you been renting a room and did you get some sort of notice?

A suggestion, join ASLOCA and visiit them, at a minimal CHF 50- per year, its is invaluable and minimal cost. This is a renters protection association, that provided legal advice by over 10 registered Geneva lawyers.