Adding People to Project Team

Hi All, Calling all WF experts! I'm trying to add users to a project, even though they are not assigned to any tasks, but I want them to be included on the project team. I've gone to People - Staffing - Add Users , but not everyone in the system is popping up as option. I've already gone into the settings and de-selected the role or group needed. (see uploaded screenshot) So my question is- why do some folks appear when I type in names, and others don't??? I've compared the user settings for one person who appeared, vs. someone who didn't- and I'm not sure what the criteria is..... tks! Karen Karen Rutz Harvard Alumni Affairs and Development