mongoose add array of ids to a object - node.js

Question

i'm trying to save a group with references to users and ranks. users and ranks is arrays of id's. However when i make a post request it seem to say that Group validation failed. What am i doing wrong with my references?
router.post('/:users/:game/:ranks/:age/:quantity/:usercount', function(req, res, next) {
var params = req.params;
var ranks = params.ranks.split(',');
var users = params.users.split(',');
var group = new Group({
users: users,
game: params.game,
ranks: ranks,
quantity: params.quantity,
age_above: params.age,
userCount: params.usercount
});
group.save(function(err, object) {
if (err) {
res.send(err);
} else {
User.findByIdAndUpdate(params.id, {$set:{group:object.id}}, {new: true}, function(err, doc){
if(err){
res.send(err);
} else {
res.send(doc);
}
}).populate('group');
}
});
});
User schema
var userSchema = new Schema({
_id: {
type: SchemaTypes.Long,
required: true,
unique: true,
index: {unique: true}
},
name: String,
birthday: Date,
country: String,
image: String,
group: { type: Schema.Types.ObjectId, ref: 'Group'},
games: [{
game: { type: Schema.Types.ObjectId, ref: 'Game'},
rank: { type: Schema.Types.ObjectId, ref: 'Ladder'},
userName: String
}],
friends: [{ type: Schema.Types.ObjectId, ref: 'User' }],
created_at: Date,
updated_at: Date
});
Error message
{
"message": "Group validation failed",
"name": "ValidationError",
"errors": {
"users": {
"message": "Cast to Array failed for value \"[ '10210697939629739' ]\" at path \"users\"",
"name": "CastError",
"kind": "Array",
"value": [
"10210697939629739"
],
"path": "users",
"reason": {
"message": "Cast to ObjectId failed for value \"10210697939629739\" at path \"users\"",
"name": "CastError",
"kind": "ObjectId",
"value": "10210697939629739",
"path": "users"
}
}
}
}