Display a grid of user groups for notifications

I'm facing this problem at work where I have two variables Category and Creators, both of which are independent.

Now, when someone uploads a file in some category I have to send notifications to certain other users of the system.

I'm currently building an interface for admins to select which users get updated when a certain combination of Category-Creator gets uploaded.

Right now I have a grid of Category-Creator with each cell showing a comma separated truncated string of user names of those people who get notified when a file in this Category-Creator combo is uploaded.

If you click on a cell a modal opens up where you can edit the list of users for that cell. Is there a better way to do this?