Member account
Choose an account in the Member account list to designate a member account that Chat Service uses to restrict or deny access to the channel. A member account can be associated with a Windows NT group, or it can contain one or more users that you designate. When you choose a member account, the Authenticated clients only check box is automatically selected, which forces the authentication of each account member.

You create a member account from Accounts under the chat server in the left pane. For more information, see the "How To" topic in the Microsoft Chat Books Online, titled "Creating a Secure Channel."