Protect entry so that only a "relatedTo" user can see it I would like to protect the viewing of an _entry.html page so that only a user related to that entry can view it.
You could loop through the users and compare them.

{% set authorized = false %}
{% for user in entry.authorizedUsers %}
    {% if user == currentUser %}
        {% set authorized = true %}
    {% endif %}
{% endfor %}

{% if not authorized %}
    {% redirect 403 %}
{% endif %}