Hi marcozambi. You haven't done anything wrong, Flarum's features are still being fleshed out. User management is something that needs more attention, and will be reconsidered after the first stable release.
The first issue you mentioned seems to be a problem. New users should be redirected to the forum home page, not a blank page. There is a lack of success message, as you mentioned, but the screen they receive doesn't seem like the right one. I'd recommend we look at this further. Can you attempt another registration, screenshot the page you are directed to, and check the error console (F12 on most browsers) to see what errors there are?
As far as the group IDs, I believe every user with an ID greater than 0 (which is a guest) is assumed to be part of the member group. If the group was literally assigned in the database, it would probably take up space unnecessarily. You can see this in the Core coding here if you want to check it out for yourself.
