Group Regstration - show on invoice the names of each person registered

In testing, we see that on the invoice the actual names of the individuals registered via group registration don't show on the invoice.
I have inserted this value into the invoice template with this value:
[GROUP_MEMBERS_NAME]

But, all you see on the invoice is that same value, not the actual names (see attached screenshot of invoice).

Am I using the wrong value? What do I need to add to the template to list the names of group members registered?
Is there also a setting somewhere I need to adjust so that this will work?

Thanks in advance, Vicky