you could do like:

var usernames = ['abc', 'def'];
async.map(usernames, function(username, callback) {
    User.findOne({username: username}, function(err, user) {
        if(err) {
            callback(err);
        } else if(!user) {
            callback('could not find user with username ' + username);
        } else {
            callback(null, user.id);
        }
    });
}, function(err, userIDs) {
    if(err) {
        //...
        return;
    }
    //...
});