Usually if you want to add a resource to a project task, that resource should first added to project. Then only you can add them in to the project task. But you can add resources directly to a project task. For that the project record, which has the corresponding project task, there will be a checkbox ‘DISPLAY ALL RESOURCES FOR PROJECT TASK ASSIGNMENT’ in the preference subtab.
By default, it will be unchecked. If you check this preference, you can directly Assign a resource, who is not a resource in that task’s project. After assigning the resource to the task, then those resources will be automatically added to the corresponding project assignee list.