Clips and Groups don’t seem to have any features setting them apart, which is confusing.
To differentiate them, I think Clips should have instances and the ability to auto-update, and therefore they show up in the library while Groups don’t. By auto-update, I mean changing one instance of a Clip automatically applies those changes to all instances in the entire project. And like in Flash, you can also edit them by double-clicking its entry in the library. Clips would also have an option for whether they’re bound to the timeline (like Graphics in Flash) or not (like Movie Clips in Flash).