Collaboration is a very important part of creating worlds in Horizon. While in the create tab, press the “i” icon on your world. Then press manage collaborators button. Here you'll have the option to add editors and testers.

Seen above is the “i” icon located next to your world.

Seen above is the manage collaborators button found on the world page.

Seen above is the manage collaborators tab where you can add friends as editors and testers.