There's no easy way to create a mail group like GMail. Only way it can be done in Outlook online is to create a group and the people in it. If I try and "copy" someone already in Outlook to the group it can't be done. In GMail I can "label" each person or business with a specific label so if I later want to send a "group" email to the people in that label I can do it in one email easily. I also keep all my contacts in one group regardless of their labels.