The PGR is not a "group" in any typical sense of the term.
You agree to their rules, which are reasonable in every way - Buy a patch (Optional I think)
If they think there is the possibility of a disruption to a military funeral they notify you
If you elect to show you arrive bringing a large American flag on a flag pole & dressed in a respectful manner, - There is no requirement that you show up
If there is a demonstration you stand between the demonstrators & the bereaved family, keeping you back to the demonstrators