code: function(usernames, rolename) {
      if (_.isEmpty(usernames)) {
        return Promise.reject(new Error('One or more usernames must be provided'));
      }
  
      if (!_.isArray(usernames)) {
        usernames = [usernames];
      }
  
      return Role.findOne({
        name: rolename
      }).populate('users').then(function(role) {
        return User.find({
          username: usernames
        }).then(function(users) {
          role.users.add(_.pluck(users, 'id'));
          return role.save();
        });
      });
    }
docs: add one or more users to a particular role
  TODO should this work with multiple roles?
  @param usernames {string or string array} - list of names of users
  @param rolename {string} - the name of the role that the users should be added to