You are able to Request to Join or Join Office365 groups depending on their privacy levels.
1. Navigate to https://outlook.office.com
2. Click on the small "Group" icon down the bottom of the left hand pane. (Group icon looks like two small people)
3. Search for the name of the group you would like to join in the search section up the top
4. Once you've located your group, choose the "Request to Join" option and the owner of that group will be notified of your request.
5. Once you have been accepted into the group you will receive an email notifying you.
