So to create the user in the database MyDb for a login named myAppLogin you just issue command like this:
USE MyDb sp_adduser myAppLogin
Starting from now the login myAppLogin can start using the database. The user can be removed using the following code:
USE MyDb sp_dropuser myAppLogin
What is very important - you cannot remove a user when it is "in use", so there is an active session in the database using this user, however when the session was not found, the user will be removed.

