An integral part of my classroom is peer feedback. While Google Classroom makes it very easy to create and collect assignments, making student work accessible to other students is more difficult. Here is a possible work around to help you get students collaborating together.
Create an Assignment
Each time you create an assignment a folder is automatically generated in the teachers Google Drive. Call this assignment "Class Share Folder" or something along those lines. The advantage to this is the folder is automatically created in your Google Drive for the class you want to create a share folder for.
Set the due date to be the last day of school.
Copy Student Email Addresses
Instead of sending the email you only want to copy the email addresses. Click on the email of the first student in the email group. Hold down the shift key. While holding down shift, click on the email of the last student in the email group. This should select ALL of the email addresses.
Control C to copy the email addresses.
Go to Google Drive.
While you can go to your Google Drive, open the Google Classroom folder, open the class folder, locate the Shared File... Google Classroom makes it easy to connect directly to the folder you just created.
Share the Folder
Share the folder by locating the breadcrumbs at the top of Google Drive. The last item in the breadcrumbs is the shared folder. Click on this to reveal a drop down menu of options for the folder. Choose "Share..."
Paste Email Addresses
Students Add to Drive
Sub folders take on the permissions of the parent folder. If you move an assignment folder in Google Drive to the shared folder this will give all the students in the class access to the entire folder with editing rights.
While I can not set a folder to be "Can Comment" I do not stress too much about students messing up other students documents because they can always be reset through the revision history. I also have had multiple digital citizenship conversations with students prior to creating a shared folder with them.
For an assignment you've created in Google Classroom, go to that folder in Google Drive. (See steps above). From the breadcrumbs click on the arrow of the folder to reveal folder options. Choose to "Move to..."
Move to Shared Folder
Move the entire folder to the "Class Shared Folder."
Students are now able to locate the assignment folder by going to the shared folder in their own Google Drive. They should be able to locate the assignment folder inside the shared folder and comment on other student work.
Move the Assignment OUT
Once the students have had the opportunity to provide peer feedback move the folder out of the shared folder. This will remove student access to the documents in the folder in one swoop.
If the students have turned in the document the teacher is no longer owner of the document and can not set sharing permissions. Students should give feedback and make corrections BEFORE they turn in the assignment to Google Classroom.