Fetching "Pending" Users Related To Entry Within Plugin
As always, spent hours trying to figure this out and came upon the answer within an hour after posting the question on Stack.

Here's the working code:

    // get entry
    $entry = craft()-&gt;entries-&gt;getEntryById($entryId);

    // get ALL users related to entry
    $users = craft()-&gt;elements-&gt;getCriteria(ElementType::User);
    $users-&gt;status = null;
    $users-&gt;relatedTo = [$entry];
    $userIds = $users-&gt;ids();