It's simple. first go to options and configure a bit (display offline friends, your nickname etc). Then whenever a contact is playing a game you'll see that, and if MP on which server and if you own that game you can click the join button and follow him in game
You will also be able to see which Teamspeak or ventrilo server that person is on
In the servers tab you'll see the 10 servers you visited last with xfire. Great if you want to join one of those again. You can also add those to favorites.
xfire can of course also be used to send instant messages to others
by adding swbattlefront.net you'll see them in friends of friends, recommended to add em to friends then