I'm pretty certain that a guest must book, pay, and complete a stay through airbnb to be able to give a review. There is no reason your friends can't do this...i personally know a few of my returning paying guests. Have them do a booking inquiry and give them a special offer...there may be a minimum but I think its around 15. They will have to pay the airbnb fee of course. It's a great idea to practice on friends and family...especially if they ham it up with odd requests for you to manage.