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.