There are a couple ways to enable collaboration on Google Sites. The first is to click the "share with others" icon in the upper-right corner of the Google Sites editor and then enter your students' email addresses. The other method is to share your Google Site as an assignment in Google Classroom and allow students to edit the file. Both methods are demonstrated in the following video.
In this video:
- How to share a Google Site via email.
- How to share a Google Site through Google Classroom.
- A suggestion on how to manage shared Google Sites.