[Parody]

Going by their descriptions in the Categories list:

Cast Lists group characters in an out-of-game way, while Groups group characters in an in-game way.

For example, you might use a Cast List to contain all of the NPCs that appear in a published module. (An out-of-game arrangement.)

In this module, you might have a Group topic talking about the Evil Conspiracy going on in the adventure, and instead contain the members underneath the group. (An in-game arrangement.)

You could do both by containing the Group under the Cast List.

Another possibility is to use Relationships with Groups to show membership/interested parties instead of containment, using a Cast List or some other method of organizing your characters. It's up to you. :)