How do I add items in a group, as you can see in the picture below, How can I add "Favorite" and "Inbox", "Drafts", "Trash" under it.
OutlookBarGroup is a HeaderedContentControl so the content can be anything. In that specific sample, when you click on the xaml tab you will see that the content of the Mail group is locCtls:XobSampleMailContent. Searching in the FeaturesBrowser for that class, you should find it under the Controls folder. That control is just a UserControl which contains 2 expanders (one for Favorites and one for Mail) and those expanders each contain a listbox with listboxitems.