An untested example of async.each to decorate a list of users.

gistfile1.js

// populate users with User.find, etc, then inside that callback:

var results = [];
async.each(users, function(user, cb) {
  Foo.find({"user._id":user._id}, function(err, result) {
    // note: return isn't returning anything valuable, it's just a shortcut to leave the function to avoid ugly } else {
    if (err) return cb(err); 
    
    if (result === true) {
      // whatever this is...
      user.foo = 'bar';
    }
    
    results.push(user);
    cb(null); 
  });
  
}, function(err) {
  // if you have err here, one of your callbacks was called with a non-null first arg.
  if (err) return cb(err);
  
  console.log('your users have been modified and the results are here:', results);
  res.send(200, results); // or some callback, etc
});