As educators, we need to be intentional and make use of every opportunity we have to meet the needs of all learners. Deciding how to seat students is just that...a decision that we make.
Grouping students is a tremendous opportunity, one of the most important, that we get. And, just like all decisions, they can become incentives for struggling learners.
There are so many ways to approach seating. The Kagan Model used to be a popular structure--based on student ability and performance. I'm not sure how relevant it is anymore. My suggestion is to group students in groups of 4 or 3.
Now, this gets into a whole other discussion of how to design seating. My suggestion for that is to ideally provide different types of seating options: pairs, threes, fours, and individual. I like seating to be driven by student choice and fluid, like most of my instructional decisions.
Of course, if students can't handle sitting where they choose, you simply move them until they can make better choices.