Something like this should do:
Remove-MgGroupMemberByRef -GroupId 08900da5-xxxx-xxxx-xxxx-5af3b2f62a16 -DirectoryObjectId 584b2b38-xxxx-xxxx-xxxx-c9766cb4791b
where you have to provide the GUIDs of both the group and the user you want to remove. You cannot provide either values, such as UPN, so use the Get-MGGroup/Get-MGUser cmdlets as needed beforehand to obtain the GUIDs.