Latetly I have added Groups (Roles) and the involved logic to the tool, so that now access to the project can be controlled. Users who are members of a group which belongs to a certain project have write access to the project. Other users have only read access to the project. If a project is marked as being private, then others won't see the project at all. An exception to these rules is the Admin user, which has access to everything. Additionally, the user who has created the project always has write access to the project.
Secondyl, I also did a code review to streamline the code and to eliminate old or unused code.