How are Custom Friends Groups Used
and also explain how to use them from the web site once they are created. And maybe mention that you have to log out and back into the clients to use them, if you do. I don't know, I only use the web client, but you probably do. So somone who knows should decide whether to include that.
If this were included then the FAQ would cover all that is vital about friend groups and when coupled with the who can see my journal entries FAQ serves as a decent guide to how friends stuff works.