What is the recommended approach to automatically share an object when a related user field is populated?

Implementing Apex Managed Sharing code is the best approach when you need to automatically share an object based on a related user field being populated. This method allows for programmatic control over sharing records and provides the flexibility to define complex sharing rules that cannot be achieved through standard sharing rules or criteria-based rules.

In situations where sharing needs to be contingent on dynamic conditions—like the presence of a user reference in a related field—Apex Managed Sharing can be tailored to evaluate these conditions and adjust sharing accordingly. This capability is especially useful in more intricate business models and security frameworks.

Other options may not address the requirement effectively. Criteria-based sharing rules can share records based on specific field criteria, but they are limited to predefined conditions and do not provide the dynamic execution needed for custom logic related to user fields. Similarly, standard sharing rules apply at a broader level without the specificity required for unique scenarios. Sending a workflow email notification does not share records; it merely informs users of changes without managing access or permissions, failing to fulfill the goal of automated sharing.