To add or remove members of a 365 group, start by navigating to https://outlook.office.com/groups/home in your browser.
1. Click on the name of the group you want to edit
2. Click the "Add Member" button to the right of the settings button:
You should get a pop-up screen* allowing you to add new members. You can do so by typing in the new member's email address and clicking on them. When you've added the members to the list, click "Add" at the bottom to finish:
To remove members or promote certain members to owners, click "View all members" in the top-right of this pop-up. You can then use the "X" next to a member to remove them from the group or the drop-down arrow to change their membership type:
*Occasionally, these pop-up windows may glitch or stall. Refresh the browser tab and it should resolve the issue.
See Microsoft's documentation for more details.