The Response Group feature is new to R2 (released Feb 3rd, 2009) and provides basic call routing capabilities within the Office Communications Server environment.
At Catapult, we are using the basic hunt group feature to provide an easy way for our employees to reach our IT Department.
Before you can create a Response Group workflow, you must first create a contact using the command-line utility rgscot.exe. (technet documentation located here).
Then, creating a hunt group is fairly straight-forward. You just select the contact that the rgscot utility created, and select the basic options you want.