My question is should we add the spa shock before or after every use?
Either one works fine. The book recommends before use, and that makes sense. The shock is gone very quickly, and keep in mind that it is simply an oxidizer - similar to hydrogen peroxide.
Also should we add the shock daily even if we don't use the tub?
First of all, there really SHOULDN'T BE a day in which you don't use your spa, but setting that aside for a moment, this is best solved by trial and error. I don't add shock unless I'm using the tub, and (true confession) mine may go many days between uses.
I was told just to add the Spa Guard Sanitizer only on start up to activate the Fresh Water stick. Is that the only time I need to add it ?
Yes, a chlorine shock on startup is a good idea, but it's mainly to get rid of anything lurking in the pipes of a new spa. Since you are 'coming over,' I would say that a shock couldn't hurt, but isn't a must. BTW you have it backwards: the MPS (shock product) is what kick-starts the Silver Ion cartridge. You will want to do a chlorine shock once a month, and after a group exits your spa.