To import into your scene go to File>Append, navigate to .blend files folder, open the file, open "Collections" folder, then double-click on the name of the group and append.

To link into your scene go to File>Link, navigate to .blend files folder, open the file, open "Collections" folder, then double-click on the name of the group and append. Then press Object -> Relations -> Make Proxy or Make Library Override. You will see the menu where you need to find "Son-rig"