Note: When you remove a member from a private group, it takes 5 minutes for the person to be blocked from the group (after membership changes are fully replicated among domain controllers).
1. Browse to the Office 365 admin center.
2. In the left navigation pane, choose Groups > Groups.
3. Select a Group.
4. In the details pane, next to Members, click Edit.
5. Next to the member you want to remove, click Remove.
6. Click Save to remove the member.