You can use add-in commands in your Office Web Add-ins to extend the Office UI. For example, you can add buttons for your add-ins on the ribbon or on selected contextual menus. We recommend that you watch this Channel9 video for a deeper overview of the feature. When added to the UI, add-in commands look and feel pretty much like native Office UI, according to the version of Office where they are running. Pretty cool, huh? This allows you to create add-ins that are easily accessible and efficient to use.
Figure 1. Add-in with commands running in Excel Desktop
Figure 2. Same add-in but loaded in Excel Online
To get started with the preview, see our samples site on GitHub, which has quick instructions to get you going. We also have documentation on MSDN that explains the preview in detail as well as a complete reference for the add-in manifest with commands.
~ The Office Extensibility Team