One one of the Windows 7 computers go to the control panel and open the HomeGroup page (or use the start > search and type homegroup). At the bottom of the page you will see a section titled Other homegroup actions and in that list you will see 'View or print the homegroup password'.
You then need to enter the password in your new computer. I don't know precisely how to do this with Windows 8 as I have not yet upgraded but this page should see you through:
http://windows.microsoft.com/en-gb/w...tart-to-finish
Focus on the section about joining a home group.
Ian