code: function createUser({ username, group="public" }, done) {
    return getGroup(group, function(getGroupErr, theGroup) {
      if (getGroupErr) {
        return done(getGroupErr);
      }
  
      if (!theGroup) {
        return done(new Error(`group '${group}' not found`));
      }
  
      theGroup.members.add({ username });
      return theGroup.save(done);
    });
  }
docs: Create a new user
  
  @param {Object} details
  @param {String} details.username
  @param {String} details.group
  @param {Function} done - done(err, user)