Send email to users after node update
But my problem is when I edit this room and I add New members, I want
  to send an email only for new.


As hook_entity_update mentions


  Get the original entity object from $entity->original.


$entity-&gt;original gives you access to the values before the update.

So all you gotta do is compare the original members to the updated members to find out who are the new members. 

$og_members = array();
$og_members = $entity-&gt;original-&gt;get('field_members')-&gt;getValue();

$up_members = array();
$up_members = $entity-&gt;get('field_members')-&gt;getValue();

$new_members = array();
// use array_diff() to find the difference.
$new_members = array_diff($up_members, $og_members);