WordPress + LimeSurvey = A perfect combination of blog fully capable of managing surveys.
NOTE: Do not set 'debug_mode' to true until database connection error message is gone!
Whenever a registered user(having an account on WordPress setup) login, this plugin check whether the user exists in LimeSurvey install or not. If yes, it can either update the password at LimeSurvey end or do nothing. If user doesn't exist in LimeSurvey database, it adds the user as per his/her role in WordPress.
If the user is anyone except 'admin', he/she may or may not see a 'SurveyPress' menu tab in dashboard which lists the Active surveys which user can take if they wish to do so!