If I was doing competition I would probably not let anyone ride who was inexperienced due to the time, energy and expensive put into the preparation process. However, right now I love to have other people ride our 2 horses. My 8 yr. QH mare came to us just out being trained for NRHA competition. She was highstrung and all she knew was to work, work, work (I don't think she liked being part of the "competition production line"). She'd never been out of the arena. No trailriding or fun and games. She had no pleasure in her life. If brings a big smile to me that now I can see her having fun in all aspects of her life. She has the ability to size up a new rider before they even put their foot in the stirrup She knows if they are experienced or have never been on a horse before in their life. This past year she has had classes of school kids, my kids friends, a Scouting group and some of my adult friends who are "fraidy cats".
I do keep them limited to the arena and the ride accompanied with a light snaffle or just the halter. However, the most important part is they are all having fun and learning about horses while she is learning about people.
Our other QH gelding spent 7 years as a lesson horse and he goes with anyone who comes to ride with us. He is a super trail horse and teacher.
Here is "Lady" with some Scouts in our arena.