If that lemmy user was found out to have invited someone they didn't know, from a random website like lemme, then they would be banned and everyone they've ever invited would as well.
Other than being invited by someone who is already on the site and you know personally the standard way is to do the interview process. The interview is mostly questions about music quality and music files and serve to make sure that you can read and apply that knowledge so that they don't have to moderate as much if you try to upload shitty rips of music on the site.
I exclusively grab FLAC. Never 24 bit. I downloaded a few to test and couldn't hear a difference. There are plenty of people who care about mp3 though.