SpaceAdvocate.com is the individual outreach arm of the Coalition for Space Exploration, whose members are companies, professional societies, and advocacy groups with a stake or interest in space exploration. While it's mostly focused on government-based space efforts such as the Vision for Space Exploration, the Coalition does include some private space companies (SpaceX, XCOR) .
SpaceAdvocate.com has resources to help you keep track of key issues in Congress, to identify and contact your elected officials, and to explain the benefits of space exploration to others. Their three-page PDF handout highlights the various benefits of space exploration and includes the helpful reminder that NASA's budget is less than one percent (0.7%) of the total Federal budget. And as I like to point out, those NASA dollars are not actually spent "in space" - they are spent here on Earth, on jobs, technology, education, research, and preparation for the future, among other things. I can't think of many government programs that give us better value, except maybe the NSF.