Error in SQL Server in network programming

I have 2 databases. The first one is created by 开发者_开发问答SQL Server Express and the second one is created programatically.

I can work with them easily but the problem occurs when another user wants to login to them. I tried to add user to my databases with Create user[Domain\User] for login ... command.

This command worked for the first database but for second one I receive this error:

"The server principal 'Domain\User' is not able to access the database 'DBName' under the current security context."

or this error:

login failed for this user

Note that my program is used in network.

please help me

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Just a guess: If the user and the server are in the domain the network want be any problem. You must add the login to the instance and assign it to a database along with the roles for the user.