how do i get my iTunes off 'guest user' & on to another user on same computer? I accidentally synced my phone for the first time under 'guest user' rather than my intended user?
If you already have all of your iTunes contents in your "Guest user" you can transfer your iTunes library contents from your Guest user account to an external hard disk using this tutorial:

http://www.ilounge.com/index.php/articles/comments/moving-your-itunes-library-to-a-new-hard-drive/

Then transfer back the iTunes contents from your external hard drive back to your regular user account.