// Result
/*
[
    { tag: 'bicycle', user: [ 'Tom', 'Bill', 'Sue' ], count: 3 },
    { tag: 'car', user: [ 'Sue', 'Bill' ], count: 2 },
    { tag: 'motorcycle', user: [ 'Sue' ], count: 1 }
]
*/

// ==================================================
var responses = [
    { user: 'Sue', tag: 'car' },
    { user: 'Sue', tag: 'bicycle' },
    { user: 'Bill', tag: 'car' },
    { user: 'Tom', tag: 'bicycle' },
    { user: 'Bill', tag: 'bicycle' },
    { user: 'Sue', tag: 'motorcycle' },
    { tag: 'motorcycle', user: 'Jack' }
];

result =[];

ty = (d)=> {return d.tag == responses[i].tag;};

tr = (v,p) => {
  return {'tag':v.tag,'user':[p.user].concat(v.user),get count(){return this.user.length;}}
}

for(var i=0;i<responses.length;i++){
if(result.length == 0){
result.push(responses.filter(ty).reduce(tr))
}else if(result.length >0){
if(result.filter(function(d){ return d.tag == responses[i].tag;}).length == 0){
  result.push(responses.filter(ty).reduce(tr))
}
}
}
console.log(result.sort(function(a,b){return a.count-b.count;}));