SurveyMonkey only supports the "authorization code" grant type, implicitly stated in the example code. Note that we are in the process of updating this OAuth code to give a much broader and detailed picture of how our OAuth works.
You cannot use SurveyMonkey's API to send in responses btw if this is what you were planning on doing - it is a read only API. You can see the list of supported requests on the left sidebar here.
When you complete OAuth, you are given an access token to the user's account who completed OAuth - i.e. you can access all their surveys and responses.
Respondents to a survey do not have an account on SurveyMonkey - they are represented as objects retrieved via the API method get_respondent_list, which accesses a specific survey's respondents. This survey is one of the surveys belonging to a specific user's account - i.e. the user account that completed OAuth.