Well if you can just do it without telling them, that would be the best.
I was sharing a BED with my cousin at the time, and I didn't tell her!
She went to sleep very early every night, so I just waited until she went to sleep. Then I went in the bathroom, got everything set up, and very quietly got into bed. She was up and gone before I got up, so it was no problem.
ha ha ha ha - she was sharing a bed with a girl who had bags of pee taped to her ankles!!!! (I did tell her a couple of years later. She is into alternative health, too, so she wasn't upset. Actually, she laughed).