Display records in order according to the quantity you have in a table

Question

I have two tables in the database, one is saved all the groups and the other the members of the groups. I want to make a page of Popular Groups, would display the groups that have more members in order of quantity. Because it’s two tables I don’t know how I could do it. What I’ve written so far:

<?php 
//Seleciona todos os grupos
$sql = $pdo->query('SELECT * FROM groups');
while($row = $sql->fetch()){

//Conta a quantidade de membros que tem em cada grupo 
$sql_count = $pdo->query('SELECT * FROM group_memberships WHERE group_id = ?', 
             [$row['id']]);
?>