Put an @  sign in front of their name, with no space between the @and their name.  It will give you auto fill.

type @pause for a box to open, start typing their name, then choose from the Chris's on the list.

Chris Mamone 

One thing to add, if the autofill doesn't initially work (I believe it only populates the first six people that you're subscribed to or on the comment thread after you start with the @ symbol) , try using the beginning of the last name.

I see some good suggestions here.

Carol Williams is exactly right.  Type the @ symbol and start typing the name of the person you wish to tag.  When you see their name on the drop down menu, click on it.

The answers you're seeing below with the @ are correct.

All the good folks below have given you the how to's. Good luck with it.

You hit the @ symbol and start typing their name.

hit the @ sign and after you type their first three letters of their name, it sould pop up.  Just click on it and continue.

Chris - Well, it looks like you have received the answer to your question. Typing the @ symbol and then start typing the person's name (no space) until it shows up.

Hi Chris, sounds like you have been given some good advice.  Good Luck!

thanks for asking, because i didnt know prior, i was using my lazer kryptonite pen to write it in, whew!!  more importantly , its nice to be recognized. good job chris