Hi Andrew! Welcome to Moodle and always feel free to ask "novice" questions (because they are usually excellent questions).
You can't have guests take quizzes, but you can create a generic account and multiple people can take the quiz using that account. However, when they take the quiz it will display the previous attempt results on the opening page which confuses folks.
If you want students to take a quiz which won't impact their grade in the Moodle gradebook, then just set the point value of the quiz to 0.
Or create an adaptive quiz (allows them to check their answers and change their answers during the quiz) with no penalty assessed for each guess. That way students will eventually get all of the questions right (this can be a very good learning tool).
If you can install the Feedback or Questionairre module on your site then that can be used to create an ungraded "quiz" which is more of a survey. I believe you can set these up to allow guest attempts, but there might be issues. Since Moodle is an LMS it expects that you are dealing with known logged in users (students) and its tools are set up for that scenario.
You might look at Google Docs creating a form if you want to embed a guest access survey in your course.