Distribution Groups
Login to https://outlook.office.com/owa/
Click gear icon select view all outlook settings
Go to General then distribution groups https://outlook.office.com/mail/options/general/distributionGroups
Office 365 Groups
Login to https://outlook.office.com/owa/
on left side Groups select mange groups and will take you to https://outlook.office.com/people/group/owner
Select required group and click on Members tab and remomve or all ones your require