Drilling down into an Entity Reference Field in Rules Actions
In my scattered experience with rules, a limitation on what data you can access is usually a safeguard because the data has not been confirmed to exist yet.  In the Rules conditions, create "entity has field" condition twice.

1) Rule Conditions > Entity has field
Data selector: node
Field: field-associated-brief

This is your entity reference field, and gives Rules the comfort of knowing the reference field is usable. Once you have the above conditions, rules will load up the referenced fields from your node

2) Rule Conditions > Entity has field
Data selector: node:field-associated-brief
Field: field-breif-recipients

3) Now the action is able to use the field confirmed in the conditions

Even if you do not think the conditions should be required, it is good practice to confirm you have the data before you try to use it.  Here is another person who had a very similar issue