Allow multiple groups to view comments

One of our users has asked if we can permission more than one group to view comments. For example, they have separate groups for Project Managment, Developer and User roles. Some of the comments are for internal viewing only - i.e. for PM and Developer - so they would like to be able to restrict permissions to only those groups. Currently this can only be done on a project-wide basis.

For the moment, I have added the PMs to the developer group so they can see comments permissioned for the developers, but it feels like this would be a useful feature going forward.