Push to array within subdocument in mongoose

I'd like to push to an array that's within a subdocument in Mongoose/MongoDB.

Here is the schema:

var UsersSchema = new mongoose.Schema({
    user: String,
    stream: String,
    author: String,
    tags: Array,
    thumb: Number,
    added: Number
})

var ContentSchema = new mongoose.Schema( {
    title: String,
    url: String,
    description: String,
    text: String,
    users: [ UsersSchema ]  
})

I'd like to push an array into the UserSchema.tags array for a specific users sub-document.

I have tried this and several variations: http://ift.tt/1aovKIX