To Activate Users who have been previously Archived, click "Users" from the Left Navigation. Next to the search box, click Advanced Filters.
This will take you to the Advanced Filter popup window. Select "Archived" in the State box. Click Search. This will filter the list to all Archived Users.
When the list changes to the Archived Users, click the check box to the left of any Users you need to Activate. Click the Manage button on the top right of the User list. This will open a panel on the right of the screen.
Click the Reactivate button on the top right corner of the panel.
Lastly, confirm the total number of archived users to be reactivated.