A Hogfriend is a person who you get along with that you want to talk to again in the future. A person can be listed as your hogfriend by going to their profile and clicking "Add Hogfriend" at the bottom of the page. You can check out your hogriends at anytime by clicking "hogfriends" in the "interact" section of the navigation bar to the left of any page. Once there you will see a list of all your current hogfriends. To owl any one of them, simply click the envelope next to their name. Any hogfriend that is listed in yellow is currently online. You can also tell how many hogfriends are online at any given time by checking the number in this icon at the top of your screen on any page:

Hogwarts Extreme (3)

Additionally, if you click on that icon, it will bring up a list of which hogfriends are currently online.