As a SH, TM, PMA, I can join a project with the private code
Stakeholders (SH) knowing the project private code can join the project proactively. They don’t need to wait for the invitation from the Project Manager (PM). Managers can always remove stakeholders or disable the project private code:

Stakeholders can also join the project with the mobile application.

Team Members (TM) knowing the project private code can assign themselves to the project (work package #0). They don’t need to wait for the assignation by the Project Manager (PM). Managers can always release a TM or disable the project private code:

Project Manager Assistant (PMA) knowing the project private code can assign themselves to the project to assist the project manager:

Frequently Asked Questions
Who can join a project using the private code in PMPeople?
Stakeholders (SH), Team Members (TM), and Project Manager Assistants (PMA) can join a project using the private code without waiting for invitations.
What happens after someone joins with the private code?
SHs are added to the stakeholder register, TMs are assigned to work package #0, and PMAs can assist the Project Manager. Managers can still remove or release them if needed.
Can private code be used in the mobile app?
Yes, users can use the PMPeople mobile application to join projects using the private code.