The two parts of the question don't make much sense together. First of all, you do not define your own types via the public API - you define them by bootstrapping a content model XML file at startup. See content model tutorials.
Do you want to create a custom type for representing users / groups? Otherwise I don't understand the second part at all. Typically you do not need to create types for users / groups since Alfresco already provides its own and only suports these. You can of course define aspects which you can use to enhance the user / group nodes that Alfresco creates with additional metadata.
