People are seeing each other when they’re guest of a space and haven’t been added at least to one team or project in common (which seems to be the case for your colleagues). If you can see them it means that you’re either a full member of this space or you’er a Guest who share at least one team or one project with other folks!
You can learn more about this in the following article: Working with guests in an organization • Asana Product Guide and in this other forum conversation: "Private user" for collaborators/Assignee - #4 by Ivan_Arsenov