"You automatically attended the local Church, joined the local PTA, and so forth. In contrast, to join an organization such as the SCA you have to go out of your way to join up. The result is a fair number of people left on the sidelines who are either too timid to make the leap or ignorant of where to jump."
With due respect, I think you're oversimplifying a bit. On the one hand, my experience with the SCA, like Wren's, is one of a great deal of interest in recruiting. Come across a group of guys in armor whacking each other with rattan swords? Start a chat with one who isn't fighting, or with one of the people standing nearby watching or chatting with one another. Many will explain, or give introductions. Joining is a quick bit of bureaucracy from there.
One the other hand, unless someone literally sees and drags you in (like what happened to me) or you befriend a member from elsewhere (also my story; it's complicated), it's easy to get overlooked. People get distracted by other things and don't always see you.
I think what ultimately hurts groups like these worst, though, is the size of our society and the humanity of the groups' members. If you don't even walk through the right park at the right time, or the right empty classroom at the right time, you may not see them or realize they exist. Also, the guy supposed to spot you and answer questions may be lazy and/or lacking in knowledge (volunteer organisations of ANY stripe takes what it can get help-wise). The leadership may get caught up in internal feuds and forget business or drive away members. One member may commit financial or legal crimes that bring the authorities down on the group as a who. I've seen one group whither because one guy was their rock; he died, and they drifted apart. Yellow pages and other maps aside, most of these things can and do easily happen with a more traditional group.