こんな感じ？
var my=function(){
var that = this;
this.arr = [];
return function(n){ that.p(n); return that; };
}
my.prototype.p=function(n){ this.arr.push(n); return this;};
my.prototype.get=function(n){ return this.arr[n];};
var a=new my();
var c = a({id:'hoge',age:20});
a({id:'fuga',age:18});
var b=new my();
var d = b({id:'moge',age:30});
b({id:'muga',age:38});
c.p({id:'nuga',age:48});

console.log(c);
console.log(d);
console.log(c.get(c.arr.length - 1));