If you need to setup Lync 2013 to start automatically when users login to their PC using Group Policy follow the steps below.
Open Group Policy Editor and create a new policy or edit existing one.
Go to -> User Configuration -> administrative Templates -> System -> logon -> Run these programs at user logon
Add Lync 2013 .exe path
“C:\Program Files\Microsoft Office\Office15\lync.exe” /fromrunkey”