Getting current logged in users blog url in adminbar on main site

Hi, I need some WPMU pro to answer this quick question I have. I just want to show the current logged in users multisite domain (they should only have one multisite each) in their admin bar, even when they are viewing the main site (pro-sites plugin requires site owners to go to the main site to access the checkout pages etc.)
At the momemt the code I am using just cant seem to get their website on my button.
I am using the wp admin bar -> add_menu to add a custom button in the admin bar....
All help as usual, greatly appreciated!