Head to the Inside Out Project to start participating. The steps are simple: create a portait, upload it and when it is printed and sent back to you, post it in your community. From the site:
People can participate as an individual or in a group; posters can be placed anywhere, from a solitary image in an office window to a wall of portraits on an abandoned building or a full stadium. These exhibitions will be documented, archived and viewable virtually.
The project already has plenty of life as you can see from the uploaded photos already posted.
Worth watching to learn more about the project is the following video that follows Inside Out as it takes shape in Tunisia:
Follow the Inside Out Youtube channel to see more about the project as it progresses.