This does what I need it to using async.js:

db.open(function (err, client) {
    async.waterfall([
        function (callback) {
            client.createCollection("docs", callback);
        },
        function (collection, callback) {
            for (var i = 0; i < 100; i++) {
                collection.insert({c:i}, function() {});
            }
            callback(null, 'ok');
        }
    ],
    function (err, result) {
        console.log('result: ', result);
    });
});

By putting the async code inside the anonymous function passed to db.open the 'client' variable was visible to the first async function. Got that idea from this SO answer

This does what I need it to using async.js:

db.open(function (err, client) {
    async.waterfall([
        function (callback) {
            client.createCollection("docs", callback);
        },
        function (collection, callback) {
            for (var i = 0; i < 100; i++) {
                collection.insert({c:i}, function() {});
            }
            callback(null, 'ok');
        }
    ],
    function (err, result) {
        console.log('result: ', result);
    });
});

By putting the async code inside the anonymous function passed to db.open the 'client' variable was visible to the first async function. Got that idea from this SO answer