Messages to members (of a sphere)
By typing directly in the "Write a message" field, the message will be visible to all members of the sphere.
Private messages
You can send private messages to one or several people in a sphere:
Create a new message and mention their usernames (ex. @peter_jordan)
Each username begins with "@"
Select "In private"
Click on the "Share" button. Only the members that you identified will receive the message.
A little lock on the "Share" button confirms that it is indeed a private message that you are going to share.
Tips:
When you type "@" in the message window, a list of the usernames of possible recipients appears.
You can find a person's username by clicking on their photo in the right-hand column ("Members" section) and mentioning them there.