Usually one becomes a beta tester when invited by a designer. Beta testers are usually chosen because they have knowledge and experience of what is being tested. A beta tester is one who can test a model/route etc in different conditions to see what needs to be improved or clarified before the general release.
There is an unwritten assumption made (usually by the younger or less experienced members of the forum) that a beta tester is someone who gets models early for their own gratification. Nobody will invite anyone to be a beta tester if it is thought that the tester is someone who just wants to play trains.
As an example, the team-ALCO models and products are strenuously tested before release to ensure that they work properly, replicate the prototype in appearance, sound and performance within the constraints of the MSTS program. Such people need to be knowledgeable as to the prototype, so we have current and past enginemen as well as people who have been deeply involved in the preservation movement. This is needed to ensure that the models are released when they are ready and as good as we can make them in all practicality.
Designers won't usually pass models on before release except for a good reason.
Cheers,
Matt
It rubs the lotion on its skin or else it gets the hose again.