Bring whoever you need to get through the door. People come with a partner, a sister or a friend, and we have no objection to somebody sitting downstairs for the first two or three.
They do not come into the room. Not because of a rule for its own sake, but because you will edit yourself, and the editing will be invisible to you.
Couples work is different and is the one place two people are in the room by design. If you want to bring a partner to individual sessions because talking alone is too much, say so on the call and we will discuss it properly rather than turning it down flat.