Laela - I have only used Perusall a few times. Here is what I understand.
When you set up the course, you can say how many students should be in a group. If you want all of them in one group, just give your full class number there. It does make that a setting for the course. But you can change it later and it changes it only for subsequent assignments.
Also, when you set up an assignment, you can assign specific students. It would seem at that moment that you could just add the whole class list for that assignment.
What seems good about ability to assign students to an assignment is that if you do peer editing, you can upload their pdf's of their work and then make an assignment for each paper and assign specific students to each one.
Also, when you are reading annotations from assignments where students were in small groups, you can select to read all annotations. It isn't all totally intuitive!
Others might have found other things I'm missing.