Installr

If we are in need of managing what versions were distributed to which testers and whether they have already started testing the app or not, we should give Installr (https://www.installrapp.com) a try, since functionality-wise it is quite similar to Diawi, but it also includes a dashboard to control who are the users, which apps were sent to them individually, and the status of the app in the testing device (not installed, installed, or opened). This dashboard is quite powerful and definitely a big plus when one of our requirements is to have good visibility over our testers, devices, and builds.

The downside of Installr is its free plan only covers three testing devices per build, although they offer a cheap one-time pay scheme in case we really want to have that number increased. It's a very reasonable option when we are in need of visibility and cross-platform distribution.