Finding and managing a group of testers

Question

I am about to release a piece of software that will be licensed with the GPLv3 (the project itself is "for fun" and has no commercial dimension in it).

The software is intended to help designing digital clocks and I know the core functionality works as - beside having unit tests - I used it for designing a couple of clocks myself. Yet, before getting the software advertised on hackers and makers sites, I would like to get a few voluntary testers... trying to use it and reporting any problems.

The only tests I did so far in my life were with fellow programmers or with the customers. So I would appreciate input from the community on:

1. Where to find voluntary testers (is there a well-known website that escaped my googling perhaps?)
2. How to brief them (fellow programmers and clients know a great deal on the software they are going to test... should I try to brief in depth these "unknown" testers or should I contrarily try to let them "on their own"?)
3. How to structure the tests (I'm not asking what to ask them to do here, but if I should structure my test in some specific way: for example in more phases, like having one group to make a test, then fix bugs, then having a second group test again... or collecting data in a particular way [e.g. with a questionnaire with scores rather than with free-form e-mails])
4. How to give maximum visibility to their contribution (Again: I can't afford to invest also money on this project, but I would be happy to give them as much glory as I can to them!

The focus of the test would be primarily usability (Does the GUI make sense? Is the workflow easy to follow? Are there unexpected behaviours?). But of course any bug that should emerge in the core functionality would definitively receive all my attention too!

Thanks in advance for your advice and time.