Perusall automatically groups students in the course into smaller "discussion groups," by default with about 20 students. Students will only see the comments of other students in their group, plus any threads that an instructor has started. This is to ensure that:
Each document has a different set of groups formed, so students have an opportunity to interact with different people over the course of the semester. Groups for a particular document are fixed once the document is uploaded, to ensure students have a consistent view of each document.
The number of groups formed depends on the enrollment estimate and target group size, both of which can be updated through the course settings. However, note that since the groups are fixed, changing these settings will not affect the groups for any documents already added to the course; they will instead take effect going forward for any documents uploaded after that point.
As a consequence of this, if a student is the first one in their group to comment on a particular document, then they won't see any other comments in the document even though other students in the class may have also started commenting. This is normal; they should start to see other students' comments as others in their group start reading and commenting on the document as well.
Automatic vs manual groups
If you prefer to manually place students into groups, you can do so under the Grouping Options tab of the course settings. Here you can manually assign students to groups by entering a group number next to each student in your roster. For example, to place students A, B, C, and D into two groups, you could assign students A and B to group 1 and students C and D to group 2. Students that enroll in your course after you set the groups will be placed into one of the groups at random, but you can change this in the Grouping Options at any time.
Changing grouping options
Be careful when changing the grouping settings for a course: since students only see the comments of others in their group, a change mid-course could be confusing to students since they will see a different set of comments when reopening a document after you have made the change. In particular, it is possible that a student may no longer be able to see one of their own comments in the document; if student B responded to a comment by student A and then those two students were later placed into separate groups, student B would no longer be able to see her response because she would no longer see student A's comment.