Once a user is logged in to your SQL Server instance, he may or may not have access to your databases. In order to access a database, SQL Server login of that user must be mapped to database user created inside the database. Use following command to list the logins who have access to your databases. I have used sys.database_principals system view for this information.
use DBName GO SELECT UserName = dp.name, UserType = dp.type_desc, LoginName = sp.name, LoginType = sp.type_desc FROM sys.database_principals dp JOIN sys.server_principals sp ON dp.principal_id = sp.principal_id GO