Updated site, looks easier than booking a commercial airline ticket: SpaceX PDF user guide, lots of details: https://storage.googleapis.com/rideshare-static/Rideshare_Payload_Users_Guide.pdf
Spaceflight Industries to sell rideshare business to Japanese firms - SpaceNews.com Good for Spaceflight to dump the boat anchor, but I'm a bit confused why anyone would want to buy that capability...
Looks like Planet is the first big customer (to my recollection) with the rideshare program, launching the hangar-queen Skysats that have been sitting around for a few years. Planet’s next six satellites will launch on upcoming SpaceX Starlink missions - SpaceNews.com
More microsats ridesharing with SpaceX: Exolaunch arranges rides for Loft Orbital satellites - SpaceNews.com