I'd say sensei just relates to the master/student relationship. Therefore if you are the instructor of the night you are called sensei - if not, you aren't. (I wouldn't call a high ranking student I was training with sensei, but I would call any instructor sensei).
I believe the purpose of using the term sensei is to establish who is the teaching authority in the dojo. If you have two people of the same rank at a dojo and only one of them is the instructor I would only call the isntructor sensei; otherwise the teaching could become very chaotic.