ABSOLUTELY tell your guests they will be sharing with other unknown people! People searching for a place are overwhelmed with choices and often do not read or remember every detail in every single listing. The more you can do to make things clear and comfortable the better.
I am currently hosting a young woman - here for a month on a medical rotation - who fled her Airbnb when she discovered it was basically a "pot house" with small dirty rooms and other unknown people staying there (sharing 5 to a bathroom.) Even though she told Airbnb about the problem, and that she felt unsafe, they still kept $1,000.00 of her payment, refunding her only $600.00.
We all need to go out of our way to insure that guests know exactly what they are getting, to protect the brand if nothing else. (Though I am posted on Airbnb, she found me through Homeaway.com.)