If you want to advertise for roommates or flatmates, the title of your ad should be "Flat Share" or "Room Share".

Google translate says 'roommate'. Is that the word you want? If you are sharing your room with another girl, she is your 'roommate'.

Just say "we rent a room/apartment together". Yes, the other person(s) are your roommates or flatmates. Ignore the link to cotenancy and joint tenancy. They are specific legal terms subject to a number of conditions, normally in property ownership and probate arrangements.