You are able to add and remove members from Groups you are the owner of in Office 365
This process is the same for both Windows and MacOS users
First, go to https://office.com and sign in with your St Mary's Account and Password
Once you are signed in, choose Outlook on the left side of the window
With Outlook open, you can scroll down on the left side until you see your Groups
Choose the Group you want to manage and you should see a screen similar to the one above.
From this screen, click on the # members underneath the group name
You will now have the Members window appear - from here you can:
Add members by choosing the option on the right
Remove members by clicking on the x
Change roles by clicking on the drop down arrow next to their role
If you are unable to make these changes, then you will need to contact the Owner of your respective Group for assistance.