Cannot Connect to Database by NT User Group

I am using SQL Express SP2 with Windows Authentication and Windows XP SP2.

Assigned a Windows user "UserSomeone" to an NT User group as "MyUserGroup".

Set server role members in SQL server for NT User group "MyUserGroup" as
db_creator and public.

Created a database "MyDatabase" as admin who is also a member of
"MyUserGroup".
Set role members of database for NT User group "MyUserGroup" as
db_backupoperator, db_datareader and db_datawriter.

Switched to user login that is assigned to NT User group "MyUserGroup".

With C# .NET attempted to open database using SqlConnection.Open().
Recieved error:
Cannot open database "MyDatabase" requested by the login. The login failed.
Login failed for user 'MACHINENAME\UserSomeone'.

Does anyone now why I cannot connect to this database?
Am I missing some other permission?