Microsoft was friendly enough to publish a way to join a client without the need to join the domain. It’s rather simple.
Open an elevated command prompt on the client computer.
Type: reg add “HKLM\SOFTWARE\Microsoft\Windows Server\ClientDeployment” /v SkipDomainJoin /t REG_DWORD /d 1
Now just connect your client throught the Windows Server 2012 Essentials connect website.