I can help you with the first part of your question. If I understand you correctly, you are looking to create a new service, so you don’t want to find something exactly the same?

There are some examples of volunteer and skill match websites, to help you with a model for your site, but I was not able to find any that are specifically in the field of grassroots action (though those below may list some grassroots/radical projects).

NGO volunteer sites: United Nations Volunteers Online: organisations and volunteers can register and be matched to projects:

Go Volunteer: achieves a similar aim for smaller community groups (nothing to stop radical groups from participating!) also: Volunteer Match

Libraries often create classes on demand for groups of patrons within a range of skills. For example, see the guidelines at University of Oregon.

43 Things: Not quite what you’re after, but a good example of people matching – this site matches goals to people who want to do them and people who have achieved them. People can post advice and encouragement

Good luck with your project, it's a great idea!

I would also suggest contacting the small business centre of your local public library, even though you are not starting a business. Librarians often answer questions from people wanting to know if their idea has been done before eg to apply for patents.

More suggestions from other volunteers welcome!