Communities Invites Only authentication restricts non-community members from accessing a survey link. Once this survey authentication is enabled, a shared survey link by the community member cannot be used to take the survey by a non-community member.
Go to: My Surveys (Select Survey) » Edit
Click on the Settings menu under Edit. Select Security on the left navigation bar. Select the Communities Invites Only option and save.Survey should be sent using communities send option. Respondent panel members can click open and respond to the survey when they are logged in via their community.
If the survey link is shared and opened externally, respondents will see the following error message: