Riding course

I think residential lots have become more available to be used to visitors. In my save I changed the New Appaloosa park training lot into a residential lot, and other sims can still visit and use anything on the lot without even knocking on that door. So, my sims living elsewhere can still visit to use the training equipment (and even borrow horses living there) without even greeting the sim residing the lot. I love this, it allows for me to use parts of residential lots for various community services.
PS: I think borrowing a horse will only train your riding sim's horse riding skill, and will not build the horse's skills. And it seems your sim can only enter contests with a same household horse. Bringing own horse to train on a residential lot will build skills for both rider and horse, though.