How can I display related entries selected within a Matrix field?
When you relate an element from a Matrix block, it's important to know that the relation is between the Matrix block and the related element; nor the entry (or whatever has the Matrix field) and the related element.

So, the correct way to select related elements through a Matrix field is:

{% set employer = craft.entries.section('companies').slug(segment) %}
{% set personnel = craft.entries.section('people').relatedTo({
    targetElement: employer,
    field: "matrixField.relationField"
}) %}


Update: Added a missing } on the last line.