Its a code on the back of the Guitar Hero Greatest Hits manual that lets you play some of the songs on Guitar Hero 5???
Zero-F – That is what he is talking about… not very useful answer… oh and at the moment it is only for Greatest Hits and World Tour
Anyway, I think it is very unlikely that someone will give you their code as they will want to keep it to themselves, the only way you could absolutely guarantee getting the code is to buy the game for yourself BRAND NEW as second hand versions may have already had their code used.
Hope this helped you