Consider the following scenario:
You deploy Microsoft Exchange Server and Lync Server 2010 Group Chat in a Lync Server 2010 environment.
You enable user A for Exchange Server and Lync Server 2010. For example, you assign the Simple Mail Transfer Protocol (SMTP) address of "firstname.lastname@example.org" and the Session Initiation Protocol (SIP) address of “sip:email@example.com” to user A.
You enable user B for Exchange Server. For example, you assign the SMTP address of “firstname.lastname@example.org” to user B.
You add user A to a chat room by using the Group Chat administration tool.
User A signs in to the Group Chat client, and then user A chats or performs some activities in the chat room.
You disable user A for both Exchange Server and Office Communications Server 2007 R2, and then you remove the account from all chat rooms.
You change the SMTP address of user B from "email@example.com" to "firstname.lastname@example.org." You enable user B for Lync Server 2010 by using this new SMTP address and the SIP address of "sip:email@example.com."
You try to add user B to a chat room by using the Group Chat administration tool:
Unable to save the member list:   Role <Member> for <ma-chan://contoso.com/a9844a0d-528c-42aa-a4aa-70f5dcc2ed49> already contains >These principals are already defined on the node: <sip:firstname.lastname@example.org>
This issue occurs because the entry for user A is removed unexpectedly in the Group Chat database after you disable user A for Lync Server 2010.
To resolve this issue, install the following cumulative update:
2514984 Description of the update for Lync Server 2010 Group Chat: November 2011
Note You must delete the old account in Active Directory Domain Services (AD DS) before you install the update.