Can't seem to edit page title in og/users/%/add_user
This is what helped me in this particular situation, but I'd be open to mark another answer correct if anyone had a better option inside of a hook_form_alter(). 

// Force Title on the Add Users OG form. 
if ($form['#id'] == 'og-add-users') {
    drupal_set_title('Add Users');
}