You are able to restore an archived volunteer, as long as they have not been permanently deleted.

To unarchive a user, do the following:

Go to Organizer Dashboard

Click on the Volunteers tab

Scroll to the bottom and click on see archived users

To restore an individual or multiple users, check the boxes next to the name(s). Then in the actions drop down, click on Restore UserRun and Confirm. 

OR to restore a specific individual, you can click on the name of the archived user

Click on the Permissions tab and click on Restore Volunteer.