I've created a join table of values that represents a set of data unique to a specific employer. I want to validate the names of these values so that they are unique only within a specific employer, not across all employers, and I don't know how to do this.

Create a text field, set it to auto-enter the employerID and the value, with a separator such as a pipe

employerID & "|" & value

Set this field's validation to be unique. This will prevent a duplicate from being saved, however it is still possible for the user to attempt to add a duplicate.

To prevent this, consider using one of the "dwindling value list" techniques, which removes an option once it has already been entered.