First, find the applicant's entry on your Applicants tab and click the thumbs up next to Hire.
You will be prompted to fill out some information about the applicant.
The applicant will automatically be added to your Users page under the Account tab.
If you made a mistake and need to remove this applicant from your Users, you can immediately click Undo this change on the pop up if you're still on your Applicants tab. You may also notify the referrer (if applicable) of the new hire at this point.
If you need to undo the move after navigating away from the Applicants page, you may undo the hire at any time on the Users page by finding the Undo Hire link next to the applicant's name.