Users have only
db_datawriter roles that allow to access database, read and write tables. Within a database, one login can be used only by one user. Mind, that the login used to enter the database is already used by you and it can't be used to create a new user on this database. Therefore, when you create a new user to the database, the system will offer you to choose from the logins that aren't used by this database.
To add a new user to individual Microsoft SQL database: