I haven't tried this myself, but could you use virtual sections (one per group) to accomplish something like this? That is, just download the group membership (which is now possible) and then re-upload it to automatically populate as many sections as there are groups. Once each group is it's own virtual section, I think you could ensure that group members would only review each others work (i.e., only members within the same section). In the worst-case scenario, you might have to create dummy assignments for each section, but I THINK you could do it all within a single assignment assigned to "all" of the sections simultaneously.
Just a thought as something to investigate.
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