If someone with a user account tries to schedule, publish or edit a post and gets these notifications it means that they don't have rights yet within the project.
You might be thinking, wait, I already gave them publishing rights when I created their account. With those rights, they should be able to publish? And yes they are, but each project also has their own publishing rights. You can easily give them rights in a project. Just follow the steps from the article Starting a new project.