Hello there, @BeckyZ ...
To my knowledge, you cannot assign a module or a file only to a certain group of students. However, there may be a work-around that you could implement that would accomplish the same thing. Try this... create a module as normal. Next, create an assignment. This will be an ungraded assignment that will not show up in your Gradebook (set "Display Grade as" to "Not Graded" and make it worth 0 points). Then, in the assignment directions, create links to the files that you want only certain students to see. (This could be a bulleted list of files, or you could include additional instructions with the list of files...it's pretty much up to you.) Once you have all of that done, then it is time to assign that assignment to your group of students. There are a few ways you can do this...by individual student, by group (if you've set up groups in your course), or by section (if you've got multiple sections of students in the same course):
Save the assignment, and then repeat the process...again creating a separate assignment for another group of student with different files linked for that group. Since you are assigning the assignments to only certain people, they should only see the one assignment that you placed them in...along with the files you want them to see.
Do you think this might be a workable solution for you? Hope to hear back from you soon. Sing out if you have any other questions about this...thanks! Take care, and be well.