Now in Outlook on the Web, after searching your inbox, you will now see the option to modify your search to run the query across your groups (public and private that you are a member of).
In the past, the only way to search your joined groups was to do one of the following:
Notice while typing your query in the OWA inbox search bar that there is an option for “Search in Groups”, and click that. Historically, if you ever pressed ENTER, you would lose the opportunity to re-scope that search to your joined groups.
Alternately, if you go to any arbitrary group, the OWA search bar will then let you search that specific group. After doing so, there is then a refiner that shows up that lets you adjust the scoping of the search to “Your Groups”.
This began rolling out to First Release customers end of last week, and should be out worldwide by the end of the month (January 2017).