How to Add Users to SQL Server database

execute a CREATE LOGIN/CREATE USER statement...

http://msdn.microsoft.com/en-us/library/ms173463.aspx

delete them using DROP USER

http://msdn.microsoft.com/en-us/library/ms189438.aspx

How do you give the user roles like DbOwner?

You can add roles using sp_addrolemember. See http://sqlserverplanet.com/security/add-user-to-role

And how do you specify the domain where the user is coming from given that when I add the user domain\\user I get the following error:  

I would like to add the following user:

Mycompanydomain\myuser as opposed to just myuser.

This is not working, especially that I am passing the query through my C# code to get this work.  Please help me find a solution so I can put this to rest.  I need the following written in C#

SqlCommand cmd = new SqlCommand("Create LOGIN " + usertobeadded + " USE " + databasename + " " +"CREATE USER " + usertobeadded + " FOR LOGIN " + usertobeadded " GO", conn);

I keep on getting with the '\' sign or the ';' sign.  What do I do?

I'm not a C# developer - I code mainly in Java - but I'm guessing it might be an issue with escaping the backslash character.

http://blog.akilles.org/2008/03/12/escaping-backslash-character-in-c-net-mysql-queries/