Fetching Existing Users In Users Field Doesnt Work When entries.saveEntry Event Triggerred From Frontend
Turns out I was inadvertently "emptying" my user field on form post. Essentially I had named one of the form fields "fields[customUserField]" which upon submitting the form, was saving an empty value into the "Users" field. Then my custom event would trigger and fetch the "Users" field thereby retrieving an empty "array".

So nothing is wrong with the code below:

// 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();