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.
Important
A tester is only able to preview your world.
An editor should be someone you trust as they will have most of the same capabilities as you.
Only the world creator can publish, change world properties, and add collaborators.
You can have up to four users in build or preview mode at any time, this includes yourself.