Here is one way to do this -- there are definitely other possible variations on this.
What I am showing here is also covered in an example in the section “Create Nested Axes for Character Variables” in the JMP Help (https://support.sas.com/documentation/onlinedoc/jmp/13.1/EssentialGraphing.pdf) -- you can see a screenshot of the extra X-axis drop zone in Figure 3.16.
The trick is that there is a hidden drop zone in the X-axis that lets you add a second categorization variable on top of or below the variable that you already have in the x-axis. Just drag that second variable over the variable that is already in the x-axis and then move your mouse slightly up – you should see a smaller blue drop zone appear above the main x-axis drop zone, as well as a small zone appearing below the main zone.
Here I have my continuous Y in the Y zone, and my categorical X in the X zone:
Then I add on the boxplots:
And then I drag on the second categorical X variable into that zone that appears when I hover over and then slightly above the X drop zone:
(Sorry about the “Where(IndividualWithinTrt = 1, 2, 3)” extra stuff – the example I had on hand has 13 groups within each of the four treatment groups, and that just looks way too messy.)
Just for extra clarity, I’ve now colored the four main treatment groups. You can see that each main group has three subgroups, which each have three points in them.