I just had this happen. 2:50am, I was woken up by the two of them. I deliberated for about 5 mins.
Knocked on the door, to which he answered and she was out of site. Told him clearly:
"I don't mind if she stays but it will incur an additional fee in line with my charging of other guests." (Literally 10AUD)
She came out and started telling me about herself, to which I said "I don't care. It's 2:50am"
She decided to leave. I apologized to him, for interupting.
I also told her to flush the toilet (as she had left a bowl full of unflushed paper).
Essentially it's two guests. So add the extra guest fee.
Consider a rule that stipulates this in the room 'no extra guests between such and such hours' or 'extra guests will incur a nominal charge of $##'