When your are creator of the project
When you create a new project, by default, only you have access to it.
You can then choose with whom you want to share the project and define access rights for each member, but it is not mandatory. You can very well have some projects for yourself only and some projects shared with others.
Even in the case of a subscription with several beneficiaries, each one can choose whether they want to share their projects or not.
To learn more, see the article dedicated to project sharing.
Know who can see the project
To see who a project is shared with, go to the project settings: Project> 
“Settings” icon in the top right corner of the window. Go to the “Collaboration on project” section.
There, you can consult the list of members having access to the projects, as well as their access rights.
You can modify this list only if you are creator of the project, or if you have administrator rights on it.