Once the first user logs into a new chromebook enterprise enrollment is disabled so Before you activate your chromebooks go to:

1. domain management → Settings → ChromeOS → Device Settings
2. Set Enroll Devices Automatically to Allow Devices to Enroll Automatically

1. Power on the Chrome device and follow the on-screen instructions until you see the sign on screen. Do not sign in yet.
2. Before signing in to the Chrome device, press the key combination Ctrl-Alt-E. The enrollment screen appears.
3. Enter the user name and password from your Google welcome letter, or the user name and password for an existing Google Apps user on your account. The user associated with the account becomes the owner of the Chrome device.
4. Click Enroll device.