Click to see a larger image
To share your Address Book, go to preferences and go to the "Sharing" panel. Check "Share your Address Book," and click on the + button to add other Mobile Me users (look for e-mail addresses that end in @me.com or @mac.com) from your address book.
Click the "Send Invitation" button and each person on the list will receive an e-mail that contains a link that allows them to subscribe to your address book.
To subscribe to another Mobile Me user's Address Book, go to the menu "File / Subscribe to Address Book…" and enter then e-mail address of the Mobile Me user. The other Mobile Me user must have already added you to her sharing list for you to be able to share her addresses.