Sorry, that one doesn’t have any hooks built-in that would allow you to modify the behavior, and it relies on a group field that was added to the sheet table in the main plugin just for this extension.
The main part of the Groups plugin is not overly complicated, and if you are decent with coding, you could probably modify it to handle assigning more than one group to a sheet, but that still requires a significant number of changes that you wouldn’t have been able to do with simple filter or action hooks anyway. You would have to change the single-select box to a multi-select box for groups on the admin side, and then change the processing of that form to store those as a list of groups (separated by commas or some other character), and then when retrieving that data, build that list into an array, and then change the function that checks if a sheet belongs to the group to see if the group is in the array of groups (instead of just checking for one specific set value). I’m sure there are plenty of other things that would need to be changed as well.
So, not extremely difficult, but tedious because of all the functionality that would need to be changed.