Updating an entry through mongoose without ObjectId

I have a Mongoose schema defined as

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var userSchema = new Schema({
    user_id: String,
    event_organizer: [String],
});

module.exports = mongoose.model('User',userSchema);

Now, I have a function where I wish to add this User's Id to an event. Ofcourse, this event is already present in the DB.

function addUserToEvent(user_id, event_id) {

}

How do I add an event_id to the user's event_organizer array as defined in the schema?

It is possible that the array is already populated and I need to append the id, not reset it.

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