If the survey participant (internal or external) clicks the link in the invitation email, after they login they will be taken right to the survey page. This is by design and the default Invitation email template has this link already added. If you want to customize the content of the survey invite email you can do that as well but make sure you have the right tokens added to the email. You do this in the summary page of the survey (edit document first), then go to Summary section there in the actions menu you will see a customize email, select the Publish Event, Invitation for Participants email template.
I've attached some screenshots.
Let me know if this helps.
Regards,
Maria