Amazon is really on a role when it comes to development tools. There was already a facility to allow other accounts to access a skill that is still in development, yet it was difficult to use for multiple users and also required the tester to navigate the Amazon Developer website. It was also a little wierd for Alexa developers, as all the language in the prompts referred to mobile apps.
Amazon recently announced a new beta testing facility for skill developers is available! You enable the testing feature it from within your skill dashboard. You must fill out both the Publishing Information and Privacy & Compliance sections of your skill to start the beta process. You can then add users, who will be invited to your test. There are management tools to add and remove testers as well as ability to view testing metrics ont he number of active testers.
This beta test system is itself in beta test (that's a good one!).