I find numerous examples on how to add a new group to an existing ribbon, and this works just fine.
What I cannot figure out is how I can add new controls to an existing group on an existing ribbon. Say I want to add my own command to the "Proofing" group on the "Review" tab.
I'm developing this in VS2010 for Office2010, but I guess the same approach would work on Office 2007 as well.
Any pointers or help is appreciated, or if it's not possible to do (without too much hacking) I can live with that as well.
Unfortunately, this is not possible. You may only add controls to custom groups in Office 2007/2010.
The built-in groups are really a different thing than the custom groups that you may add. For example, you will see for example that the built-in groups such as the ones for font and paragraph formatting behave differently with respect to resizing the application window.