Looks like the animation editor got launched. The workshop has changed to enhance this new feature.
You can use the 2 small buttons on the left. (Under the delete button)

I've tested it right now and I'm unable to remove a piece from a group, so there may be a bug or something.

EDIT: You can also use drag and drop in the "graph" panel.

Those two buttons from the left are not useful at all. I saw them in the first place. My problem is that i couldnt make simple operations. For example i select some pieces, then i group them, and after that lets say i want to add more pieces tot that already created group. With those buttons you can`t do because it creates a new group from that newly piece.

But, still, i call this resolve because the drag and drop function you are saying. This works.

But is drag and drop the only way? Because if you have large models, it takes a very long time holding down the mousebutton and releasing it at the right time, to move a new part from the bottom of the graph panel to the top into an existing group...! I really really miss the add/remove function we used to have! 😦