When you create a family group, you can manage privacy and online safety settings for family group members on Xbox One, Xbox Series X|S, the Xbox Family Settings app on mobile devices, and Windows PC.
Any organiser in the family group can add new members to the group. To add a member, follow the steps on this page.
READ MORE   :   www.microsoft.com/link
 
Note that if the person you’re adding to your family group is already part of another family group, they must be removed from the current family group before following the steps on this page. 

Sign in directly on the Xbox console

Using a Controller

How do I find out what Microsoft account is linked to my Xbox?

Go to Settings > Account. Scroll right to Your Information, and then select Account Security. The Microsoft account that’s associated with your Xbox gamertag is shown on the right side of the screen.