Check with local RV dealers. Groups looking for new members will often use dealer bulletin boards. There are actually quite a few groups out there, not all affiliated with Good Sam, FMCA, or brands like WIT.
I've also met new groups by watching who else comes into the campgrounds and RV parks our group has stayed at. Many groups put up banners, and you will find them using club rooms and rental shelters.
Most groups are open to new members, because this is needed to keep them going. Attrition tends to be high among groups mostly composed of retirees. E.G. we've lost five members in the past three years (from a 12 used to be 20 family club) and others have reached a point where they can no longer handle RVing so that we have to find places to go where they can get cabins or nearby hotel rooms. If you've recently retired and are physically active, you would be the new members some club out there is seeking.