Query below returns list of users in current database.
User grants a login access to the database
One login can be associated with many users but only in different databases
select name as username, create_date, modify_date, type_desc as type, authentication_type_desc as authentication_type from sys.database_principals where type not in ('A', 'G', 'R', 'X') and sid is not null order by username;
Those results show that there are 5 users in current database.
You can vew database users using SQL Server Management studio. Expand Database -> Security -> Users branch in Object Explorer. This option shows also roles and Windows groups.