When you write a message, just click on the single player little image you can see right above what you are writing.
Something like this will appear: [player=XXX]
At that point, put the player's ID number in place of the XXX and you have your link.
Works the same way for teams' and games' links. Just click on those little images on the right of the player's one instead.
"BuzzerBeater, the best simulation game of real market economy" (with a cool live basket viewer).