Let’s say that you have a GPO that is scoped to a specific security group. If you add a computer to this security group, you would normally need to restart in order for the computer to see that it is now a member of this group. To bypass this, you can delete the system’s Kerberos ticket and run GPUpdate. The computer will magically see its new group membership without a restart.
Source : DeployHappiness | Quick Tip: Add Computer to Group – GPUpdate without Restart