Group Chat and Office Communicator 2007 R2 are both features that are shipped with Office Communications Server 2007 R2. 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 are displayed and can be updated within Group Chat. Office Communicator also updates its Contact Lists with these updates. If Office Communicator 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 Office Communicator.
When Group Chat is not the primary IM client, there are two differences to note. 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, you double-click on the participant's name. This opens an Office Communicator chat window as shown below.
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 Communicator 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 Office Communicator 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 Office Communicator or through Office Outlook.
Group Chat determines whether Office Communicator is installed at startup. It does this by examining the registry. If Office Communicator is installed, the Group Chat IM capability is turned off. IM conversations can still be carried out by clicking a participant. However, this opens an Office Communicator window. If the user is offline, Office Communicator is launched and the user is prompted to log in.
Whether Group Chat or Office Communicator is the preferred client is determined by the DisableIM key in the registry. If IM is disabled, or if Office Communicator is not installed, then Group Chat is determined to be the preferred client. Otherwise, Office Communicator is the preferred client.