To join the a2a ("affiliate to affiliate") program...

1.  Log in at the SFI Affiliate Center:
www.sfimg.com

2. Go to the a2a tab at
www.sfimg.com/Home?tab=a2a and complete the registration information that is asked for. 

That's it. You can now begin using a2a immediately.

For more information about how you can put a2a to work for you, see:

www.sfimg.com/LaunchPad/Day24