One option here is to upload you CSV data with a custom column that you've flagged for the group you want (i.e. set the 10 players you want to 1 and everyone else to 0 in a new column called "GroupFlag" or something.

Then when you go to Advanced Options > Groups > Create New Group, you can then filter the players on that column and only add those to your group. It's not a perfect solution, but I do something similar.