One of the rules of Karate etiquette, in the club I belong to, is never leave a black belt standing without a partner when you are told to get a partner. This is extremely disrespectful to the black belt and to the Sensei teaching the class.
Even if you are brown belt or below and you already have a partner who is brown belt or below you need to make sure that the black belt without one has a partner. This means you might have to leave your partner to go with the black belt but it will certainly show that you respect the higher rank student and maybe keep the entire class from doing many push ups.