def append_group_inst(nodes, group_name, data=bpy.data, group_creation_func=None, use_fake_user=False, parent=None, custom_color=None):
    group = None
    try:
        group = data.node_groups[group_name]
    except IndexError:
        if group_creation_func:
            group = group_creation_func(data, use_fake_user)

    if group:
        gp_in = nodes.new('ShaderNodeGroup')
        gp_in.node_tree = group
        gp_in.name = group_name
        gp_in.parent = parent
        if custom_color:
            gp_in.use_custom_color = True
            gp_in.color = custom_color
        return gp_in
    else:
        return None