You could be sued for anything - but to protect myself this is what I have done:
1. Post your state's equine liability sign so they HAVE to see it upon entering property (front gate perhaps?)
2. Get a release where it states in plain english that riding is dangerous and rider acknowledges that fact. Also make certain that rider MUST wear an ASTM/SEI-approved equestrian safety helmet with a chin strap. Be certain you leave one in the tack room in case they don't have one so they don't have an excuse.
3. Both you and rider must sign release - in front of a third party who is not related to either of you.
4. Include both parties have initialed that you have walked them through some basics - proper way to catch, lead, clean, tie, and mount horse. Also that you have provided a safe place to ride - free of trash, etc.
5. Note in release that if either party varies off this agreement it must be in writing and initialed by both parties.
Dressage is for Trainers!