Add or remove a group as a favorite
If you belong to a lot of groups and need to find a certain group quickly, you can make a group a favorite (also called pinning), which puts the group at the top of the group list in the group navigation pane.
Sign into Office 365, and navigate to Outlook, People, Calendar, or OneDrive.
If necessary, click All Apps to see all the groups.
In the navigation pane, under People, locate the Groups tab, and then do one of the following:
Click on the group name, and then select Add to Favorites or Remove from Favorites.
After adding a group to favorites, you will see a star next to the group. If you remove a group, the star will disappear.
Add to Favorites
Remove from Favorites
A group favorite is not the same thing as an Outlook Web App favorite, which is used to organize email folders.