Add plugins to their own toolbar group?

I might have the terminology wrong here, but I have a plugin I've written — which is to be part of a range plugins — which I want to add to their own group; like the "Bold", "Italic", "Underline" et cetera are in their own group.

I've tried no end of variations in the "config.js", for example:

{ name: 'annotations', groups: [ 'comment', 'bookmark' ] }