I will support as soon as possible

I think you should call canvas.addGroup , but not Group.addGroups .

But canvas.addGroup would add a new group to the canvas. That's not what i want to do.

I just want to change the parent of a group that already exist.
Afterwards the subgroup should me moved when the parent group is moved.

canvas.addGroup({
  id: 'xxx',
  group: 'xxx' // you can pass group id
})

@noonnightstorm

Your code sample would create anewsubgroup, right?
I'd like to turn an existing group into a subgroup.

Edited:
I tried your code snippet. It doesn't work.
I think the functions Group.addGroups and Group.removeGroups are really missing.

Maybe you can canvas.removeGroups and canvas.addGroups .

I already tried it but it does not work.

When i use canvas.removeGroups and canvas.addGroups the nodes won't be part of the group anymore.
That means that the nodes of the group don't move when i move the group itself.

@noonnightstorm

I currently don't get the problem with that issue as the behavior already exists when you drag and drop a group into another group.

Could you please tell me what's the difficulty?

@noonnightstorm

I would really appreciate your help in this case.
Is there any chance to get a response?