Simply head over to the Applicant Tracking Tab, run a search, then check the boxes (on the left) of the profile for the applicants you’d like to tag. 

 

Once the profiles have been selected, click 'Mass Actions' on the bottom right of the page. The option "Mass Add Tags" will allow you to tag the selected applicants at once. 

 

Note: that the tags you are adding are going be added to each profile you’ve checked.