If you are planning to migrate an existing application to Alfresco, then you might want to migrate its existing users to Alfresco as well. If you use LDAP or NTLM-based security model, then you don't have to migrate the existing users to Alfresco. Instead, you can directly connect to those user sources from Alfresco. This model is always preferred as you can manage users and groups at one centralized location and access the user information in many applications.
This is applicable if you are using Alfresco out-of-the-box security model.
Any user/group is stored as a node in Alfresco. Below actions are taken when we create a user in Alfresco.