The builds are only (supposed to be) available for any Android users who have purchased the game through the Google Play store. Based on the steps I went through, Google evidently has a system in place that allows developers to offer pre-releases or betas to selected individuals. In this case, Mojang gave a fairly wide "invite". I would imagine that Apple and Amazon apparently do not offer such a system, hence why they are excluded from participating in the build testing. And there are currently over 27,000 people that have been accepted into the Minecraft PE Test Group on Google+, so I'm not sure where you got the number 90 from.
@everyone that asks for release dates: we don't have them! We have for sure a timeline we would like to follow... but we want the freedom to say *screw that* and release weeks before or after without people complaining, if there are valid reasons. So, unless everything is pretty much finalized (ie. coming out in 1 week or so) you won't have a deadline from us, sorreh.
How to play the build versions?