Say Hello
This is an easy one 
In order to get this achievement just send someone a private message.
You have a couple of options to access the message system:
- starting in home page, you can mouseover Home menu -> click on Messages, and send from there
- open someone's profile page and underneath the profile picture you have a row of options, 3rd one is Message
- after a game, you can click your opponent name and in the bottom of the little window that pops up you have a few buttons, 2nd one is Message
There are probably other ways as well, but I am sure these are the most common / used ones.
