Supervisors are a feature of Premium Independent Projects and Premium Team Projects. To see all benefits of upgrading your Project to Premium, click here
Only Admins can promote other users to Supervisor.
To do this, navigate to the Admin tab within a Project:
The setting to add a new Supervisor is available here:
Clicking this will allow you to promote a Project Member to Supervisor:
Supervisors are a customizable role. Supervisor access levels can be registered independently of General Members. This is very useful in ensuring seamless communication between General Members and the Project Administration.