The purpose of Groove being "anywhere" collaboration almost heralds the need for a relay: Users will almost certainly have to go through firewalls to collaborate, or deal with users on different subnets. A relay also keeps offline users in sync by shooting lasting changes to their client when they come online.
One of the benefits to handling your own Relay servers is that you can design your layout to assign specific servers to certain users (based upon location and network bandwidth). You can also structure multirelay servers per user to allow for fail-over in the design of your deployments.
Groove Data Bridge: A Data Bridge is used to connect your Groove clients with enterprise databases (a back-end SQL server, perhaps) and applications. For example, a Data Bridge can help to connect your Groove clients with some of your other servers that might handle files or forms, such as a SharePoint Server. This server is completely optional and one that we didn't see in the Enterprise Services offered by Microsoft.
Groove Data Bridge can be used to maintain central copies of workspaces. Usually this is for two types of scenarios: to centrally manage workspace creation and invitation, or as a backup. As an example of the first scenario, you might set up the automatic creation of workspaces in response to specific activities or events in a departmental server-based system. Custom development, using the Groove Web Services API, is required to do this.
For backup, the Data Bridge server can automatically archive workspaces. This gives you a centralized alternative to having individual workspace members make archives from their PCs. But this cannot back up all workspaces -- only those in which the Data Bridge server is a member. That means that users will need to invite the Data Bridge server into their workspaces -- unless the workspaces were created centrally by a custom application associated with the server.
"It is possible to deploy Groove Data Bridge on-site, while still using Groove Enterprise Services for the Manager and Relay components," says Abbott Lowell, senior product manager on the Office Groove Team at Microsoft. "For example, an organization using Groove Enterprise Services might opt to deploy the Data Bridge in-house for its Archive Service (to create workspace archives)."