If we use HRBP as a reference field for the example, we will do the following.
Instead of adding the specific people as group members, you can add HRBP people by adding the condition "HRBP is assigned as":
This means that every person that is assigned as someone else's HRBP will automatically be added to this group.
In order to give the HRBP members the permissions only over the people they are assigned as this employee reference, you will add the condition under Access rights "HRBP is viewer":
