Mongoose : Error referencing sub schema

My user model

'use strict';

const mongoose = require("mongoose");
const Schema = mongoose.Schema;


var UserSchema = new mongoose.Schema({
    id: String,
    username: String,
    firstName: String,
    lastName: String,
    initials: String,
    password: String,
    age: Number,
    dateJoined: Date,
    contactNo: String,
    email: String,
    about: String,

    groupId: Number,
    adminMode: Boolean,
    simpulPoints: Number,
})

//Define model for user
const User = {
    UserModel: mongoose.model("user", UserSchema),

}


module.exports = {
    UserSchema : UserSchema,
    User : User
}

My Events Model

'use strict';

const mongoose = require("mongoose");
const Schema = mongoose.Schema;
var UserSchema = require("../models/user").UserSchema


var EventSchema = new mongoose.Schema();
EventSchema.add({
    id: String,
    title: String,
    description: String,
    organizerId: String, //Simpul admin user responsible for event
    startDate: Date, //MM-DD-YYYY HH:MM:SS:sssZ
    endDate: Date,
    group: String,
    locaction: String,
    googleMapsLink: String,
    hasPassed: Boolean,
    attendees: Number,

    registeredUsers: [UserSchema],

    groupId: Number,
    adminMode: Boolean,
    simpulAward: Number,
});

//Define model for evnt
var Event = {

    EventModel : mongoose.model("event", EventSchema),

}

module.exports = {
    Event : Event,
    EventSchema : EventSchema
}

I'm getting the infamous "throw new TypeError('Invalid value for schema Array path" error with the 'registeredUsers' field. I've followed multiple posts with the same problem and can't seem to find where I am going wrong. According to my knowledge, I've exported the schemas appropriately. Any help/tips welcome

via Arno Bornman