How to reactivate a user once archived?

To reactive a user you need to go to the people page, then to users.

Then select the filter button next to the search bar.

From here you can select disabled, under status on the filter section. Then press the green apply button.

This will bring up a list of users who have been archived, You can then find the user and select them.

This will bring up the users page, from here you tab to user details.

Once the user details tab is selected you can scroll to the bottom and select the 'Reactivate User' 

This will then reinstate them on your active user list.