There needs to be a way to add permissions to sub-groups so that we can set up consultants with access when they need to work with a specific team without giving them access to all of the processes.
I understand this would be a structural change but the current workaround of making the process group a top-level group isn't really viable longer term.
"Subgroups inherit permissions assigned to top-level groups. Inherited permission can be removed. You cannot add permissions for subgroups."
For now you could consider creating a top-level group, giving the consultants access to only that group, and create shortcut processes in that group. However they will still have access to any other process group which has the 'All Staff' role applied.