Username: specifies the user name to add add or change
Password: specifies the user name's password
Login Users and Groups specifies the list of available user names and groups. Sending an invitation when security is enabled requires selecting a user name or group to be used as login credentials.
(email invitations sent with a specific user name requires a login of the user name and password)
Select email: is used to select an existing email address to use as the user name.
Monitor: switches to monitoring mode for monitoring logged in users.
Email Address Manager: is used to manage email addresses. Used by Select email as possible user names. Used by invitation sending for possible fro and to email addresses.
Signed In Users: displayed when there are users signed in. Click to expand (>>) or (<<) hide the list of signed in. To sign out a user, select the user then click the sign out button. The ip address for the user name is shown as [ip_address] before the user name.

When security is enabled, the User Manager is available for managing users and groups. The User Manager can be used to:

• add a user
• add a group

• remove a user
• remove a group

• change an existing users
• change an exiting groups password
• monitor logged in users
  

Sending Invitations
Once a user/group is added, it is available for use when sending invitations. When sending invitations, a user/group the invitee must login as must be selected. This is how access to your iTunes Music Library is securely controlled.

The differences between users and groups
Users and groups are treated exactly the same from a security viewpoint and groups are only provide for logically grouping your users. i.e. you may have user Tom and group Family with Tom being a user and Family being a group that is used by your family. They are both login user names...