I've been following them for years. They're pretty much an open book. I wrote about them in my EPI report, "Up From Deficit Reduction" years ago. They have a pretty extensive web site.
Concord is a little slicker and centrist than they are being made out to be here. Their basic obsession is national saving. They do not oppose tax increases to finance new spending, as long as deficits go down or surpluses go up. They're not particularly gung ho about SS privatization for the same reason: their precious budget surpluses might be used up to finance a transition. The Citizens for a Sound Economy, Heritage, or Cato are much more forthright about shrinking the size of government per se.
Concord is supported by elite financial interests. Unlike many letterhead organizations, they have an actual membership -- mostly upscale retired folks, typically involved for idealistic reasons (e.g., don't saddle our kids with debt).
Let me know if you need to know anything else.
Cheers,
Max