When your site is set to offline, you might want some users, other than admin's to be able to login to the site to see the updates happening. By default this is not possible. Doctor Joomla to the rescue!
How to enable this:
- Go to System > Global configuration > Permissions tab
- Select the group you want to have access to the offline site
- Set 'Offline access' to 'Allowed'
The group you selected now has access.