Migrating users from one Forest/Domain to another Forest/Domain

I want to migrate existing users/groups in a different forest/domain to our forest/domain. Can anyone tell me how can I do this? I am also trying to introduce the new DC and through of demoting the existing DC and add that member server to the new domain. I know that if I demote the DC, it is going to remove the existing users and group, but we have applications that use some of the users and group accounts in the existing domain. Can anybody help?

Well, you can download the Active Directory Migration Tool v3.0. It simplifies the process of restructuring your operating environment to meet the needs of your organization. Download it from here - http://www.microsoft.com/en-us/downl....aspx?id=17488