On the same site, you can register only once. But, if you, for example, want to play the game Aviatrix https://aviatrixbet.com/, then you can play it on different sites. Of course, only where there is this game. But you will need to register everywhere if you plan to play for real money. Because each site will ask you for your personal data, which you will not be able to provide twice. But your friends will be able to play with you in the same casino after successful authorization.