The thing is, unless you work teaching..for a huge cooperation, or decent business type setting that can get sue for all its worth, i wouldnt worry about liability waivers.
In the case of you are just taking the friend of the friend example, the way i see it, if you need a liability waiver...you have no business teaching that person. Taking charge, issuing boundaries, issuing commands, instructing, and positioning yourself to be able to be in physical control is just a part of actual coaching for none regulars/people who you cant vouch for.