There are many ways to do that. I will tell you the straightforward one first and then give you more hints.
Go to My Home
Click on Send a PM
On the new screen that will appear, type the name of the person you need to send the message to (say dimopoulos)
Click the Add to Addressbook check box
Type your message and click Continue to send it.
The next time you will go to send a message, on that small combo box displaying the address book entries, you will see the username <u>dimopoulos</u>. You have successfully entered it in your addressbook /images/graemlins/clap.gif
That's it
Now there are shortcuts. Because we are all lazy... (hmmm) what you can do is use the Who's Online screen or the profile of each user.
So for instance right now I can see KitKat being online (at the top of my screen).
I click on her name and her profile shows up.
The first link at the bottom of the profile section is Send a Private Message. Click on that.
The private messaging screen appears with the name KitKat on the recipient.
Click now the Add to Addressbook
Type your message and send it.
<u>KitKat</u> will now be in your addressbook.
Voila!
Have fun