I’ve never recruited testers before (I usually just ask friends), but I think the best practice is just to have good contact with them.
No, it isn’t required. It might be more convenient for you to make a group, but you don’t have to.
If you were to make a group, then yes. If not, just provide them with the link wherever you have grouped your testers.
You can always test them yourself to see if they work. If that doesn’t satisfy you, then just invite a few testers to your game (via Team Create) and allow them to test gamepasses and developer products in Roblox Studio.