Note: Though the screen shots are in iOS, you can also do the same thing in OSX Address Book
Step 1: Add a new field to the contact
Step 2: Select Job Title
Step 3: Fill in the Job Title with the tags you need.
START THE TAG WITH DASHES. The dash is not needed but it will help with searching short tags like "CS" which could be contained within names. This will aid in searching.
Job Title is chosen because it's one of the few fields where indexing is done but is likely to be less bothersome if filled with tags instead of the actual thing. It'd be very annoying if this was done in the Nickname field.