Okay…I finally found the article I had read before. This is an excerpt:
Airbnb says that 75% of trips made on the service are reviewed, but despite the fact that two-thirds of trips are made with two or more guests, only the person who made the booking is able to write a review. Now the platform supports multi-party reviews. The host will be able to write a review that applies to everyone in a group, and individual guests will be able to write reviews of their own.
If one of the guests don’t have an Airbnb account yet, their review will be tied to their email address. That way if they ever do decide to sign up for an account, they’ll already have a review (or several). For someone who is always a guest and never the one booking, that means he or she can start building a reputation on service, good or bad, even before having their own account."