Groups are just ordered collections of files. They may be used for convenience, e.g. when you want to let your users rearrange order of uploaded files, or just have a single URL for a gallery: http://www.ucarecdn.com/cd334b26-c641-4393-bcce-b5041546430d~11/gallery/.
Groups are immutable. Files in a group may be accessed as via group URL or individually. For example, the third image from the above gallery can be address either of two ways:
If your users makes changes to an item, e.g adding a file to existing group, you’ll get new group that consists of already existing files plus one new. Existing files are “reused”, not “duped”. You even may want to discard old group and use only new.
You’ll get better grip on this when you start using multiupload. In any case, you may not want to use groups at all, and keep track of individual files only.