code: function composeAddRemoveGroupMember({ action="add", roles="members" }) {
    /**
     * Add/Remove user from group
     *
     * @param {String} username
     * @param {String} groupname
     * @param {Function} done - done(err)
     */
     return function(username, groupname, done) {
       return getGroup(groupname, function(getGroupErr, group) {
         if (getGroupErr) {
           return done(getGroupErr);
         }
  
         if (!group) {
           return done(new Error(`group '${groupname}' not found`));
         }
  
         return getUser(username, function(getUserErr, user) {
           if (getUserErr) {
             return done(getUserErr);
           }
  
           if (!user) {
             return done(new Error(`user '${username}' not found`));
           }
  
           group[roles][action](user.id);
           return group.save(done);
         });
       });
     };
  }
docs: Compose function for removing or adding user to group
  
  @param {Boolean} [action="add"]