From user testing sessions at RightsCon 2018:

Issue: From this initial screen most people succeeded in either using the 'Select folders' or drag & drop functionality to add folders. Nevertheless, several participants had trouble finding how to add folders to the grid.

Suggestions:
Make the icons in the toolbar bigger, put text next to them to explain what they do.
Separate out Adding folder from computer, and Receiving shared folder (Adding folder from Invite code)