Joomla! 2.5: Create an Offline Group

1. Login to back-end with superuser right.
2. Click on "Users > Groups > Add New Group".
Enter a title such as "Offline Group" and choose "Registered" option for the "Group Parent" drop-down list and then click on "Save & Close" right-top button. Now that we have created a group we need to create an access level for this group.
3. Click on "Users > Access Levels > Add New Access Level", enter a title for this new access level like "Offline Access", put the check mark next to "Offline Group" then "Save & Close";
4. Click on "Site > Global Configuration" and select the "Permissions" tab. We see a list with all the groups across their permission settings (Calculated Settings). Then click on the "Offline Group" group, under calculated settings we see "Not Allowed" permission. To enable offline access for this group we need to select new setting on "Select New Setting" column for the "Offline Access" action.

We only have to change the value from "Inherited" to "Allowed".

For more info:
http://magazine.joomla.org/issues/issue-feb-2012/item/639-Joomla-ACL-Access-Levels
http://docs.joomla.org/Access_Control_List/1.6-2.5/Tutorial