You have a reference to the entities of parent1.
From which you can get the reference to the group.
From that you can get it definition - group.definition
You have a reference to parent2, and thus its entities
Simply add the group's definition into that, retaining its transformation...
Assuming parent2 is a definition...
parent2.entities.add(group.definition, group.transformation)
However, if parent2 is an instance use
parent2.definition.entities.add(group.definition, group.transformation)