Windows 8 won't allow you to connect your computer to more than one domain at a time, nor will it allow you to connect your computer to a domain and a workgroup simultaneously. If your computer is joined to a domain already, but you want to connect to a different domain or to a workgroup, you must leave the domain to which your PC is assigned. Alternatively, if you're a business owner or administrator and you want to disjoin a computer from your institution's domain, you can do so through Active Directory Users and Computers.
Via the Control Panel
Sign in to your computer as an administrator. Press "Windows-W" to open the Settings screen and then type "sys" into the search box.
Select "System" from the results to open the System window in the Control Panel and then scroll down to the Computer Name, Domain and Workgroup Settings section.
Click "Change Settings" to open the System Properties window and then click "Change" to view your domain membership.
Select "Workgroup" from under "Member of" and then enter a name for the workgroup, or use the default name. Click "OK" to leave the domain.
Via Active Directory Users and Computers
Log on to your domain controller as an administrator and then right-click an empty area on the Start screen.
Select "All Apps" from the context menu and then choose "Active Directory Users and Computers" from Administrative Tools.
Select "Computers" from the console tree and then right-click the appropriate computer in the details pane.
Choose "Delete" from the context menu to remove the computer from the domain.
To remove a computer in AD Users and Computers, you must be part of the Domain Admins group, the Enterprise Admins group, or the Account Operators group.