Firebase App Distribution can work for this, except a) you would need to manually invite all 50 testers via their emails and b) updates aren't installed automatically - the user must download the update themselves (either via an email notification link which takes them to the App Tester web UI, or they can download the App Tester app itself).

The first part probably isn't an issue, but depending on how non-technical your users are, getting them to successfully install it and then manually retrieve updates may be a bumpy ride (I'm thinking back to what I read about the Iowa Caucus disaster).