Portal Signup Shows All Projects

[Apollo]

Hello,

my setup looks something like this:

ProjectA - Channel Access: Open
ProjectB - Channel Access: Open
ProjectC - Channel Access: Open

The problem I'm having is that when a user signs up through the portal of any of those projects they have access to all of the other projects.

For example, a user signs up on "ProjectA" but sees ProjectB and ProjectC. Or someone could be signing up through the portal of ProjectC and sees B and A.

Is it possible for a user to sign up on the portal and only get access to the specific project that he signed up for? 

Like this:
Portal sign-up for ProjectA shows only A.
Portal sign-up for ProjectB shows only B.
Portal sign-up for ProjectC shows only C. 

Restricting all projects and manually adding people is not really a solution.