Move items to an existing folder:
Start by selecting the item(s) you wish to add to a folder / group by ticking the check-box to the left of item:
Next, click on the 'move to folder' icon from the top menu:
Here you will see a list of the existing folders.
Choose the folder you want to move the item to and click 'Add to existing folder'.
Note you will need to create a folder first before you can move items to it.