JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/prototype/1.7.2.0/prototype.js"></script>

JavaScript

var itens = JSON.parse('[{"user_id":"3333","username":"cacau"},{"user_id":"3333","username":"balmer"}]');

itens.sort(function(a,b){ 
    return a.username.localeCompare(b.username);
});

var letras = {};

for(var i in itens) {
    console.log(itens);
    var l = itens[i].username[0].toUpperCase();
    letras[l] = letras[l] || [];
    letras[l].push(itens[i]);
}

for(var letra in letras) {
  document.write('LETRA ' + letra + "<br />");
  
  for(var k in letras[letra]) {
    document.write(letras[letra][k].username + "<br />");
  }
  
  document.write('<br />');
}