With this release, only you can open the levels you work on. With upcoming SDK releases the level management system will be extended to allow for the assignment of levels to projects, which multiple people can work on. At that point, only users who are part of a project will be able to edit levels of a project, while all users can open levels with the Launcher. Levels which are created now can be transferred to projects later on, so none of your work will be lost.
The level management system will protect your creations against misuse by others and allow you to define who may work on your project, while you are still able to share the project to the public.