In using Open edX’s cohort feature, you can set up auto cohorts so that when new students enroll in your class, they are automatically assigned to a random cohort. This is an important distinction to make: auto cohorts are intrinsically random. It’s a great way to fairly set up small groups of students. Here’s how it’s done:
After you’ve enable cohorts, the first step is to navigate from edX Studio into Advanced Settings. 
Next, scroll down to the section Cohort Configuration
Place your cursor after the opening bracket { and type return to insert a new line. In that new line, type the following:
"auto_cohort_groups": [
"Example Cohort Name A",
"Example Cohort Name B", <br> "Example Cohort Name C"
],
Make sure it looks like this image:
You can name the cohorts whatever you like, and can make as many or few as you wish. IMPORTANT: Students will be able to see the names of cohorts.
Be sure to click “Save” and that’s all there is to it! New students will be automatically randomly assigned to one of your auto cohorts.
The screen above is located under LMS > Instructor Dashboard > Cohort Management