I just received an email this morning that testflightapp.com will no longer be active as of 2/26/2015, and that I should be using the iTunes Connect TestFlight service inste
You can use the "download over the air" functionality that TestFlight uses. Apps like BetaBuilder (http://www.hanchorllc.com/betabuilder-for-ios/) will generate the HTML and manifest file for you, you just then need to upload it to some web space of your choosing and then provide a URL to your clients. There is a requirement of the app being hosted on HTTPS now.