Before I tell you how achievable, it is just a small point of detail : On the detail page of a project, in the treatment section, you have the option to check the box "under construction". So the project will not appear in the timesheet table and no email or alert about this project will be generated automatically.
Anyway, PROJEQTOR allows a user (usually the person who has an administrator profile) to define the rights of each to create, to view and / or to modify a project depending on a given profil.
The flexibility of the software also makes it possible to modify the rights of the resource on a particular project by simply modifying the profile of the resource during the allocation to this project.
For example it can pass rights given to a project manager to those of an administrator on this project, or vice versa. This is valid for all the panel of profiles.
To define your rights the first time or to revise them GO to "access rights". IF ok and depending on them, then you just have, perhaps, to switch them on allocation of the resource to the project aimed.

Just create project, without allocate any resource : only admin can see it.
Allocate tems members (possible throught team screen) : they will see the project (and noone else except admin)

thank you for your answers Alain and Babynus but both solutions "under construction" or "no ressources" dont work.
I want my project team to see every project (even if they are not project leader or member) except a few. Then I gave the creator + permission to them.Like that by default they can see everyprojet. But then they also see Project Under Construction or without ressources.

No,
Give them the reader+ access right (then they can see all projects) and allocated them on specific projects with a profile having "manager" access right (then they'll be able to update these projects)

If I do that they can't create a new project, do they ?

Best is to allocate all users to given projects with profile that removes all rights (even read right), we call this profile "no access right"
If you have many projects, you can gather them under a single one (the parent). Allocate all members to parent with proifile "no access right", and move projects to hide under this parent.
You'll just have to allocate team member to a given sub-project to give him new rights (project leader profile for instance)

Hum. Don't find that really easy to use.
I think I will take the reader + option, Anyway Project Leader don't create the project themself today.
thanks for your time !