It was at Half Moon Bay, which is just south of SF. Check out this link
HERE . I'll bet the group I saw was from Sea Horse Ranch & Friendly Acres Ranch Horse Rentals, which is the one at location "G" on the map. If you click on G they've got some reviews. Some of them good and some bad. You'll just have to read them and decide for yourself because I'm sure some of the reviewers were crazy. It sounds like you can just show up and decide if you want to ride or not, for how long and where, and it didn't sound all that expensive, compaired to other stuff in the Bay Area.