First you have to select the items you want to move. You can hold down Ctrl (or afterwards Shift for a range) and click on the pictures in the group view on the right.
The selection of the picture currently being shown can be toggled by hitting t.
Right click the group into which the selected items should be moved or copied and select the corresponding option in the popup menu below "Marked pictures".
Right click the picture after which the selected pictures should be moved and select "Move marked after this item" in the popup menu.
Select the group you want to move by left-clicking it in the tree view. Afterwards right-click the group you want to move it into or after which it should be moved; then select the corresponding option in the popup menu below "Marked group".