Consider the following scenario:
You deploy a Group chat server in a Lync Server 2010 environment.
You add a distribution group (DG) as a member of a Group Chat room.
Users in the DG try to join the Group Chat room.
In this scenario, several users or all users in the DG do not have access to the Group Chat room.
To resolve this issue, install the following cumulative update:
2884623 Description of the cumulative update for Lync Server 2010, Group Chat: October 2013
This issue occurs because of an exception that occurs when Active Directory parses the changes of the DG.
Note Users who are parsed before the exception occurs will have access to the Group Chat room.
For more information about a similar issue, go to User cannot join a Group Chat room after you add the user to a DG that is a member of the Group Chat room in a Lync Server 2010 environment.