As of the Summer of 2018 FlipGrid is free and you can create as many grids and topics as you like.
Embedded below is a 2 minute and 53 second YouTube tutorial showing what it looks like from start to finish from an educator's point of view.
I also added a few tips especially for those that teach younger grades. The option for liking videos and number of videos viewed is definitely worth looking into. Maybe having a discussion with your class about likes and number of times watching video would be worth your while.