I think I've found a small bug in Pro 2.0.1: I can't drag a layer into a collapsed group layer when other layers already exist within that group layer.
If I create a new empty Group Layer from scratch, I can drag layers into it like so:
However, with a collapsed Group layer that already has layers within it, I can't drag another layer into it in the same way as above.
I have to expand the group layer to drop another layer in.
Not a major issue, but I'm sure ESRI like clearing up little things like this!