Explanation/Reference:
Join a domain, workgroup, or homegroup
1. Open System by swiping in from the right edge of the screen, tapping Search (or if you're using a mouse, pointing to the upper-right corner of the screen, moving the mouse pointer down, and then clicking Search), entering System in the search box, and tapping or clicking System.

2. Under Computer name, domain, and workgroup settings, click Change settings. You might be asked for an admin password or to confirm your choice.
3. Click Network ID and follow the steps on your screen.
Reference: http://windows.microsoft.com/en-us/windows-8/join-domain-workgroup-homegroup