You can achieve this by creating a 3rd table which is a “junction table”. This table acts as the “intersection” between clients & features. You would do all of your grouping and checkmarking in that table.
This is called a many-to-many relationship, and is explained here:
Right, that’s how it’s done. But you can do the data entry from any of the 3 tables… you don’t have to do the data entry from the 3rd table. Your 3rd table will always be used for the grouping, though.