Any actor who isn't a season regular is credited as a guest star. There's a few slightly different paths that can take you to the guest star section of an episode.

In the top menu, go to Overview and click on Seasons. That will bring up a list of all seasons. From there, go to the episode you want to edit, and click on the episode to expand it. In the expanded section located right under the episode overview, click on the Edit button and then click Guest Stars in the left menu.

Thank you!