Hi Fernando

The short answer is No, the User entity is a System entity so it can't be changed.

But if I understood the requirement, it seems to me that what you need is another entity that defines which users have access to which projects. This is also known as a Many to Many relationship, as 1 User may 'have' many Projects and 1 Project may also 'belong' to many Users.

Usually, this is achieved with another entity with two attributes (that identifies both the User and Project entities).

You could define a new entity named Project_Users with attributes
Project_Id (of "Project Identifier" type)
User_Id (of "User Identifier" type)

In this entity you may also add extra needed information for the relationship definition, for example if the user is the project owner or if she/he may only view the project but not change it, etc.
Regards,
António Melo