Because everyone has access to all of the other preset folders, you can only share presets from your custom "User Presets" folder, and your "Shared" presets folder.
To access these, click on "PRESETS" in the bottom menu. These 2 folders are always at the bottom of all of the main folders, and always right above the "Spin The Wheel" button.
Your "User Presets" folder is where all of your custom saved presets will live. Any presets shared with you, will be in the "Shared" folder.
Both of these folders will not appear unless you have saved a custom preset or had one shared with you respectively.
You can share a preset with someone else, that was shared to you in your "Shared" preset folder.