A group definition can be added to more than one node in a configuration, and the node definition can be added to more than one host in a configuration. This means that you can allow the same groups of applications or individual applications to run on more than one host, providing redundancy in your system. Groups are optional, but they are the only way to get applications started automatically.
A group is a set of applications that you want to start when the node is started. Typically a group would contain multiple instances of the same application. This is the way to ensure that there are enough copies of your application waiting for incoming calls. The GroupName entry must precede the NodeName configuration entry for any node that refers to it (using the Group keyword). You can add related applications to the same group to ensure that they are all started at once.