What profiles can be added to a project?

You can add public profiles you have edit access to either by being a collaborator of the manager or by being a manager yourself.

The "private" profile in that project, Katerina Zagoridis should be in the project collaborator list because it is a Geni user profile. If you take a closer look you will however see that it has a "Managed by" you entry, which tells me that the user has not set her permanent password yet, so she is not the full owner of her own profile yet, so it is difficult to know how to classify the profile. Semi-public?

There is however nothing that prevents you from listing private profiles in the project description, but you should be aware that you also make these names and profiles available for search engines like Google.

I've noticed that I can only add profiles if I'm following them; being a manager alone doesn't suffice. Is that intentional or just some weird bug I'm experiencing? They won't show up in the field if I don't follow.

You cannot add private profiles to a project.

Yeah, the ones I've tried aren't private. Oh well.

How are you adding the profiles?

The usual method is to click the More Actions menu on the profile you want to add, Select Add to Project, and then select the project.

I like to work from the project's page, so I go to the project, click "Options," click "Add Profiles," and then type in the names of profiles I want to add. If I'm not following the profile, it won't show up when I type and it won't save. If I follow it, however, it shows up. That might be the same problem Paul is encountering.

Exactly, - it is like tagging people to photos, - you either do it the natural way and add a photo to the profile, or start with the photo and try to add people to it.

In the last situation you can only enter names that is loaded as a short list on the page, which means profiles you are following or collaborating with, or you have to enter the Profile-ID and click Search more (I don't know if PID's are working on projects, but I would assume that).

Bjørn said it best "You cannot add private profiles to a project."

That's the answer, it's unfortunate but makes sense as that would make living persons too public.