Register.template.php
Pesquisar
// Are they allowed to hide their email?
if ($context['allow_hide_email'])
echo '
<label for="hideEmail"><input type="checkbox" name="hideEmail" id="hideEmail" class="check" /> ', $txt[721], '</label>';

echo '

Alterar por
// Are they allowed to hide their email?
if ($context['allow_hide_email'])
echo '
<label for="hideEmail"><input type="checkbox" name="hideEmail" id="hideEmail" class="check" value="ON" checked /> ', $txt[721], '</label>';

echo '
Para membros que foram ja registados tem que entrar no phpmyadmin query

Na base de dado do forum
update `smf_members` set hideEmail=1;