I would like to start organizing my existing events but I can't for the life of me figure out how to move an exiting event into a group. Doesn't seem to be a drag-and-drop thing.
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Select the events by clicking, and holding Ctrl.
Drag the highlighted events under the group that you just created.
It's a little finicky, but a little green line will show up when you drag them to a place they can go.