In the groups the creator becomes an admin, and there’s no way to hide that you are an admin or the creator. However, you can be automatically anonymous if instead of a group you create a Facebook page, which has almost the same functionalities than a group.
Page creators are always anonymous and their messages are shown as written by the page and not by some particular person.
Click here to see a guide on how to create a page on Facebook.
How to Use Facebook >> Groups >> How do I Hide my Name in the Admins List of a Facebook Group?