Available People
This field will contain a list of users that are available to be associated with this project.
Note regarding Inactive Users: Users that have been set to a status of "Inactive" will not show up in the left "Available People" window. They will, however, show up in the right "Plans For" window. In this way, you can un-assign an inactive user if you would like. By default, the users will remain planned even if they are set to a status of inactive.
This list can be populated / refined using the Filter feature.