A lot of people use user-defined groups to organize their assets in ways that make sense for their environment - like having groups for different locations, or groups based on the primary purpose of the assets. If you're adding a number of new assets, though, it can be a bit of a pain to discover all of them and then manually add them to the correct group.
A solution to this issue is to use group rules and tags to add the assets to the correct group during the discovery process. First, you edit the group and use the Create Group Rules option. You can create a rule that will add assets to the group automatically if they have a specific tag.
Then, when you're discovering systems that belong in a group, use the Tags step of the discovery profile wizard to add the corresponding tag to those assets as they're discovered. They will then be added to the correct group automatically.