Question
We are a lean startup, looking to test our product with a few users. Not looking at large scale traffic, 50 users a day would suffice. Anything above a 100 would be detrimental.
Answer
Paid traffic via Google AdWords will be your quickest/easiest option. Setup a couple of campaigns around highly-relevant and low-cost keywords. Target KW's that are very specific and you'll get them for less cost-per-click. (CPC)
You can turn the funnel off as soon as you get enough users for testing purposes.