- Edited
In Safari, you can control-click a tab and choose to move it to an already-established tab group, whether the tab group is open or not.
Right now, Orion has a "Move Tab" item in the tab context menu—perhaps that could be renamed "Move to Open Window," and there could be another item with the feature I'm proposing called "Move to Tab Group" (which would list both open and closed tab groups in the same order as in the regular tab-group menu).
(I see this has been requested for the iOS version.)