If you put this is your signature:
[ url=http://live.xbox.com/en-US/profile/FriendsMgmt.aspx?act=Add>=YOUR GAMERTAG HERE][ img]http://img88.imageshack.us/img88/7986/add01fs3.jpg[/img][/url]
Put your gamertag where it says: 'Your gamertag here' and remove the space after the [ from img and the space after the [ from url. People only have to click it to add it. You have to be a member of xbox.com with an account related to your current gamertag.
Hope this helps people out.
I take no credit for this.
Check mine for an example.