The Nexus 3 docs say:
The npm Group is equivalent to the component group, also sometimes
referred to as scope by npm users.
The npmjs Documentation describes scope as optional:
All npm packages have a name. Some package names also have a
scope.[...]When used in package names, scopes are preceded by an @
symbol and followed by a slash
So most of the time, the 'Group' column will be blank when browsing an NPM repository in Nexus 3. If you were to install a scoped package like @angular/router into your NPM Proxy repository inside Nexus 3, the Nexus Browse Components UI would show 'angular' in the Group column.