Multi-select listboxes are only able to work between certain types of objects. For example on a user record you could select that they have access to building a, building b, and building d. You can't have multi-select list boxes on an incident / problem / change record, which is what I think you were hoping to achieve. There is an ER to extend the functionality of multi-select list boxes which you may be interested in voting on: Allow linking objects to be created between any type of object
However in your instance it sounds like what you can do instead is to have an action called "Add affected group" which creates a collect record. When you select "Add affected group" you could have a relationship to group on this window and select a group from here. So you run the "Add affected group" action for each group that you want to add to the incident / problem / change record.
If you can take a look at an OOTB database from 7.5 then you will see there is a similar action on the Full Incident process called "Add Affected User" and this allows you to add users if the incident is marked as a "Major Incident".
Thank you Karen, your information is spot on! I had thought about making the collection, but was hoping there was another way. Voted on ER!