It seems to me there are some contradictions regarding this:
1) If you haven't added any friends and sent out any message, your inbox will show a message saying "make some friends before you can send message"
2) You can go to "all members" page, then you can send messages to anyone even though they are not your friends
3) After you sent a message, on inbox page, you can click "new message", but the recipient has to be your friend, otherwise the search returns no result even though you input the users' complete name.
After all, I want to let my users be able to send messages to anyone on the inbox page, friend or not. Is this possible? Or can we make every new user befriend with every existing users automatically? Mine is a small group of people who know each other in RL, it really isn't necessary to make them "befriend" on the website before they can send messages.