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 User, Run 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.