This morning Steam users started receiving invitations to the beta test of Mafia: The Old Country from friends on their list. In fact, there is no beta test for the new Mafia. This is a scam attack aimed at gaining access to your Steam account.
Therefore, we do not recommend opening this link, and certainly do not log in to Steam and enter your passwords on unknown sites. In fact, we advise all users to enable two-factor authentication via Steam Guard.
If you did log in to the site using the link, we recommend changing your Steam password and enabling two-factor authentication.