Hi Adam,
For beta builds, we use Apple's TestFlight program to provide access. We ask users requesting access to beta builds to submit a request in the Esri Early Adopter Community Survey123 forums. Also, note that currently (5/2/2017), there isn't an active beta build, as we released 2.0 and are still in early work for 2.1; we will be providing builds of 2.1 when ready for testing.