How to add a group into a node tree with Python How do I add an already created node group into a node tree?

I have found that this works: 

group = node_tree.nodes.new("ShaderNodeGroup")
group.node_tree = other_group.node_tree


But it creates a copy of other_group instead of just referencing it. So how would I go about just straight adding it to the node tree?
After adding the new node group node you can assign the node_group data type to the node_tree:

group = node_tree.nodes.new("ShaderNodeGroup")
group.node_tree = bpy.data.node_groups['OldNodeGroupName']