Hi there, its good to hear you have External authentication working on Alfresco 5. Would you mind sharing how you went about achieving that?
When I had this working in Alfresco 4 I used to create some properties for new users as follows:
What you need is a json array of user properties like userName, firstName, lastName, email, password to use with the alfresco/service/api/people service to create the new user with these properties populated.
Hope that helps.