Users have only public, db_datareader and 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:
- Select Microsoft SQL databases in the Databases menu.
- On the page that appears, click the name of existing database or click the Edit icon in the Controls section. The following form will show:

- At the bottom of the page click Add icon against the Database usersfield. The following form will appear:

- Enter user name.
- Choose login this user will belong to and click Add.
- Agree to additional charges if any.