When you navigate to the RDS Manager you will be presented with a way to login. Management functionality is only available to authorized users with and administrative role. Login is similar to any other expected authentication on the internet, you must provide your credentials and log in.
RDS may be configured in a variety of ways thate could affect the UI that users are presented with.
Client Managed Active Directory
In this case the client may have and Active Directory that they manage and maintain. This means that whenever user management actions need to occur (creating a new user, resetting a password, etc) the client will need to handle this through their own processes, not through the RDS Manager. It is also typical that the user will use their user name rather than an email adress to log in
RDS Managed LDAP
In this instance RDS will be set up to connect to an LDAP server to handle the managment of users. This means that the user management actions will occur in RDS. Users will typically be able to self register and there will be links in the UI to help users reset their passwords.