Take a service where you can buy places/tickets to do an activity, like enjoying a tour, or seeing a show, eating at a restaurant, whatever you want. As is common, the service asks users to leave a review after the activity, which is then displayed on the page about that activity, for future users.
Users can buy those places for several people at once, which means the other people involved will somehow “benefit” from the service without actually using it.
Does it make sense for that service to allow (or ask for) as many user reviews, as there were people involved in the transaction? (Maybe the service has a way to contact the other persons, or else it can just provide the buying user with a series of token allowing for a fixed number of reviews.)
There are services where reviews are open to anyone without having to prove you actually “used” the product, including services essentially reviews-oriented (TripAdvisor / Yelp / Google Places...), and services that do sell said products (Amazon...); but the other services usually only allow the buyer to leave one review, no matter how many people were involved and could give an opinion (Airbnb...). What is the rationale behind each of those two choices? What would make you prefer one or the other, and why not try something in-between?