A second dropdown could be added to group nodes that let the user specify if it’s an instance of an existing group, or if it’s a unique group.
All instance groups will open up to the same place, meaning you can keep your wiring much neater with no crossovers. Sort of like using a wireless router instead of having cables running all over the house.
Just like how you can duplicate group input nodes multiple times inside a group to avoid wires starting from one location and running riot all over the place.
For example, take this slightly messy node tree:
With group instances it could look like this:
Ctrl Shift D would be a good additional way to create them, which would automatically populate the dropdown with the name of the original.