Group Chat and Lync 2010 are both features that are shipped with Lync Server 2010. Because the two features share several capabilities, it is helpful to know how the two interact. The behavior of the two features depends on which one is designated as the preferred client.
For example, when Group Chat is the preferred client, Contact Lists appear and can be updated within Group Chat. Lync 2010 also updates its Contact Lists with these updates. If Lync 2010 is the preferred client, Group Chat does not display the Contact List, and because it is not available, no IM messages or sessions can be initiated through Group Chat's Contact List. However, users can double-click participants in a chat room and initiate IM sessions by using Lync 2010.
When Group Chat is not the primary IM client, there are two differences. First, a Contact List which would appear in the My Chat panel if Group Chat were the primary IM client, does not appear. Second, participants in the particular chat room you are in, still have presence information associated with them. You can still initiate an IM session with them. To do this, double-click on the participant's name. A Lync 2010 chat window opens.
Conversely, if Group Chat is the default IM client, the My Chat panel includes a list of contacts and their presence information. The Participant List remains the same as when Group Chat is not the primary IM client. However, the difference is, now when you double-click a contact or a participant, instead of opening a Lync chat session, the Message Entry Area displays messages you send to the contact or participant, as well as messages you receive from the participant. You send messages to the participant by typing the text into the Message Entry Area.
User presence can be viewed in both Lync 2010 and Group Chat. However, Group Chat cannot display the extended presence states (for example, "in a call" or "in a meeting"). These are accessible through Lync 2010 or through Office Outlook.
Group Chat determines whether Lync 2010 is installed at startup. It does this by examining the registry. If Lync 2010 is installed, the Group Chat IM capability is turned off. IM conversations can still be carried out by clicking a participant. However, this opens a Lync 2010 window. If the user is offline, Lync 2010 is launched and the user is prompted to log in.
Whether Group Chat or Lync 2010 is the preferred client is determined by the DisableIM key in the registry. If IM is disabled, or if Lync 2010 is not installed, then Group Chat is determined to be the preferred client. Otherwise, Lync 2010 is the preferred client.