Hi,
The users in your live environment will not be migrated to your development environment automatically. You will need to make the old users (live environment users) to the new dev. environment users.
Please have a read of the artical the the URL below.
http://support.microsoft.com/kb/952934
It describes different ways of moving MSCRM v4.0 deployment.
Hassan.
Hassan Hussain | http://hassanhussain.wordpress.com/