Firstly, welcome to zines! Zines are really fun for both contributors and members of the fandom. So the difference between a Zine "Guest" and a Zine "Contributor" is primarily that a Guest is specifically invited to work on the project.

This may be because a particular guest is well known for making art/writings/etc that fits the theme of the zine, or because they have a striking style, or any other number of reasons. It can vary from zine to zine. The guests also contribute to the zine though! General contributors are contributors that have gone through the application process, and are reviewed by the mod team.

Hope this helps, and feel free to ask anything else you might need!