Microsoft Office 365 - Adding/Changing Groups | WorldVia : Safe Harbors Support Center

Navigate to the Active Users page in the Microsoft 365 Admin Center: https://admin.microsoft.com/AdminPortal/Home#/users
Select the User by name
Click Manage groups under Groups:
Note, if this is a new user, you will notice there are some groups that have been automatically assigned. These are done automatically based on policies. Please do not remove them.
Click Assign memberships
Choose groups as requested by the manager and click Add.
Done! You will see this green banner when successfully added:

Microsoft Office 365 - Adding/Changing Groups Print