In Beaker, every user has a profile drive which represents them.
The "Address Book" is a way to keep track of your contacts' profile drives. Applications can request contacts from your address book, and you can use it to quickly find their profiles.
Your address book is private. Unless you give access to an application, nobody else can see it.
When you open a new tab, you can access your address book by clicking on the "Address Book" link under your pinned bookmarks.
To add a contact, click on the "+ New Contact" button.
Applications can ask for individual contacts or to read your entire address book. See the beaker.contacts API.