How can I get the email attribute from the Group Members API?

Dear Sir

I get member information from groups following this.
/groups/:id/members

However, cannot get only the email attribute.
As stated in the API reference, the email is set to public in the user settings but cannot get it.
Do I need to set my account to premium or put a special scope on it?
Please I would like to get some advice.

Kind regards

Masa