Microsoft provides several different ways how to manage the different groups types in Microsoft 365. Here the easiest way we have found so far.
In your Outlook client click on the Home tab in your ribbon (if you aren't already there) and select Address Book
In the Address Book dialog box, make sure you are using the Global Address book/list and search for the name of the group you want to modify.
Right click on the group you want to modify and select Properties
In the Properties dialog you can modify the membership of the group - as long as you are an owner of the group.