Is there a way from the SOAP/METADATA API to get the all the registered users of an enterprise using an API call with the admin's auth-token? I only require the email address, name and role of the all the enterprise users
you could either use the REST API
curl https://<INSTANCE>.salesforce.com/services/data/v20.0/query/?q=SELECT+email+,+name+,+roleid+from+User -H "Authorization: Bearer token"
or you can run a SOQL query against the SOAP API using your access token for the session ID
Details in the WebServices API Documentation http://www.salesforce.com/us/developer/docs/api/index_Left.htm#StartTopic=Content/sforce_api_calls_query.htm