Does the club have a name, is there someone who organises runs, or keeps records of members (even if they don't pay)?
Those things might make you a 'club' or 'organisation', I guess, but whether that's enough I don't know.
It's always a risk that someone will take action against you if they had an accident. Our ambulance-chasing culture, I'm afraid - if it wasn't your fault, it must be someone else's and you can get compensation!
Even if they didn't succeed, defending it could cost you a lot of time and money.