Ever since the release of SQL Server 2005 and the implementation of schemas there has been a bug when you try to assign a default schema to a windows group in a database. It has been, and still is, not possible to assign a default schema to a windows group.
The result of this behaviour is that whenever a user, who is a member of the windows group you want to add, connects to the database, SQL Server will automatically create a separate user and schema with the same domainusername as the user. And why is this a problem then? Well, if for instance you have a lot of users in the group you will end up with a lot of separate user accounts in your database which might not be what you intended when you decided to follow best practise and add windows groups to your database. Another thing is that users creating database objects has to create them using fully qualified names (not so bad actually…) The good news is that this issue should be corrected in the next version – Denali. http://connect.microsoft.com/SQLServer/feedback/details/328585/default-schema-for-windows-group