To add new members to an Microsoft 365 (M365) Group, you can use the Add-QADO365GroupMember cmdlet on the Active Roles Management Shell.
For more information on adding a member to an M365 Group using the Management Shell interface, see the Active Roles PowerShell Reference Guide.
NOTE: Azure AD does not support adding M365 groups as members to other M365 groups. For more information, see the Add member page of the Microsoft GRAPH REST API documentation.