What to do if an animal your sitting is being aggressive?

If he's not acting aggressively toward you, then for the remainder of the stay don't have company over. Be proactive on walks and avoid places where you are likely to encounter other people or can't avoid others if they appear. If you absolutely can't prevent company in your home, make sure the guest dog is safely confined from before they enter until after the strangers are gone; if you can't do that, contact the owners and Rover support to make other arrangements until the owners return.

I understand it will be a difficult stay, but unexpected behavior is just part of the job, and we often have to inconvenience ourselves for the sake of our guests. I've had to change or cancel plans with friends or family when a guest dog hasn't settled in well or displayed issues I didn't expect. After doing this job a while, I just learned to not make solid plans during a first stay with a new dog because I can't always predict how they will react to the transition. He's likely very stressed without his owners around and scared of the new people. Continuing to expose him to company isn't fair to him or to your company and can be very dangerous, so unless you're a qualified behaviorist with the consent of the owners, do not continue trying to introduce him to strangers. Management is a much safer option than attempting to work on the issue.