@Valerie1059 I'd not previously known about a "Hold Harmless Agreement" so have learned a lot already!
To be perfectly honest I would have thought that what @Lorna170 had commented, makes a lot of sense (ie re checking with an attorney or your insurance company), although I would have thought that for any agreement to be as as watertight as possible, the agreement would have to be signed by all parties?
Whilst in essence having the primary renter signing on behalf of all guests, attendees, etc should be sufficient, one also has to (imho) consider the individual(s) who might feel/believe that by the primary renter having taken away the rights of the individual, his/her rights have been denied to them.
Therefore to be on the safe side (of the law), it's always best to have guidance and advice from those who practice it.