This is perfect for teachers and students looking for mentors outside of the direct school community.
In my high school science classroom, currently there is a ton of unidentified minerals and rocks in wooden boxes... These donated "gems" are useless until I know more about them. Being teamed up with a geology mentor could make a huge difference for me and my students.
Started doing some research into well-established senior organizations in Southern California. Through my mother, I learned about a busy, well-organized, volunteer-run senior travel club. http://arcadiatravelers.com/
They're a model senior-city government collaboration. They have flyers and bulletins that are distributed on a seasonal basis to keep people informed along with a volunteer run phone line that is staffed Monday-Friday to help.
My next thought is... what kind of prototype could help move this concept forward?
A web-based network is definitely an interesting option. On one end, established groups could document their strengths, histories and successes. On the other end of things, people and service providers could access the network to explore social options... This makes me wonder if "Meet Up" groups are already doing this informally.
In terms of attracting new and existing groups, I keep thinking Kenny's reply... Bulletin boards are awesome. Profiles in the local paper, also, awesome. Maybe, at a local level, a list of groups could be tacked on to catalogues already published by community centers, community colleges, and parks departments.