Populate Select Based On Relationship Data

Another question about populating fields. I have a CPT called Job. This is set to have Relationships to 2 different user types (Client and Sitter). All that is fine.

I'd like to be able to populate some Selection Advanced fields based on values from the Relationships. For instance I'd like one SA field to be filled with a list of all instances of the pet_name field (in a cloned group) for the Client user type. Another SA field is to be populated by all users of type Sitter connected to the Job.

How would I go about doing this?

Thanks.