Under custom investments and you are trying to assign resources using the Assigned Resources field, but resources only show if they have been added to the staff.
Expected Results - All resources are displayed and available
Actual Results - Only resources that are assigned to the Staff on the project appear
Component : Resource Management
This is caused by a setting in the Project. In Classic View, go to Project>>Properties>>Settings. There is an Assignment Pool field that if defaulted to Resource Pool, will bring up all resources.
To get the behavior that you are requesting, follow steps below -
Now, you will see all resources under Assigned Resources when creating any new investments under that custom investment.
Note - It appears that any investments that existed before changing the default Assignment Pool will continue to only show the Team Only.