i am not sure i understood your question still i will try to answer that - For making service activity to be used by authenticated user, you need to go security tab of the service activity. You need to check the checkbox "Require authenticatoin to run".

Also, you need to have "requires authentication" checkbox in the Service Package. There you need to give authentication type.

But coming back to your question, I feel once you give the username and password of that access group (service package) requestor will changed to authenticated.