Group Owners - How to add or remove group membership
Log in to office.com and click on Outlook. Expand Groups, then select Manage Groups
Then click on Owner to see what Groups you are the owner of. You can manage the membership of these groups.
Click on the Members tab, or see all Members at the bottom of the members.
If you click on the Members tab, you can then Add new members to your group. You can also change the role of members to owners if you want to. This will give them the same level of administration that you currently have.