Hi Athena, automating with such a WFR is possible but may become cumbersome if resource allocation changes a lot. If the managers are typically the same pool of managers that will oversee resources in most/all projects, maybe consider adding these reviewers automatically on any new project upon creation, instead of making it a resource based trigger. That is if the intent to have them follow the project. If it is resource management - they have visibility into their directs by default.
Hi Athena,
Here is a sample WFR (on Resource Link) for achieving this:

You will want this to have a ‘Run Time’ of ‘When a new record is created’ so it only fires when the resource is added. You can add evaluation criteria to filter out certain situations you don’t want this to trigger on, for example, (1) when Direct Manager field is NULL
I would recommend checking the ‘Ignore if already exists’ at the bottom of this action as many Resources on one Project could have the same Direct Manager.