When the R Gruppe was founded, Early 911's were selling for less than a beat up SC. In fact, many (most?) of the current members owned their Early cars long before the R Gruppe was around - back in the days when nobody wanted longhoods. The club simply attracted a bunch of like-minded Porsche enthusiasts - those who were fanatical about Early 911s and that unique era (65-73) of Porsche motorsport. It has never had anything to do with how much the cars are worth.
Buying an Early 911 to become involved in R Gruppe would seem to be a strange reason to buy a car. Everyone I know would own their Early car regardless of membership in any club - it's a sickness, I'll admit that.
Here's what R Gruppe membership is all about
:
http://www.chpltd.com/911_porsche_world/roadmovie.html