Unless I put the Resource A into Project 1 then the Resource A cannot see their activities in Project 2.
Or you can affect resource A to project 2.
This a basis security constraint.
Resources cannot see activities of project they are not affected to.
Please make the selection of resources only available if they are in the project above and not all resources in the entire system.
No, this is not possible.
It mist be possible to affect Resource A to project 2 without needing to affect it to Project 1.
This also is basis security constraint.
Exemple :
Project 1 = Bad Projects
Project 1.1
Project 2 = Good Projects
Project 2.1
If I affect resources to Project 1.1 or 2.1, I just don't want them to know in wich Top project the project they work on is.
In Project 2 I selected Resource B from the entire set of resources but did not add Resource B to Project 1. Resource B could not access their activities.
I'm sure they can. Try disconnect / reconnect to retrieve correct access rights.