Your register() view needs to create an instance of the SignUpForm from forms. It should also render and return the

from flask import Flask, g
from flask.ext.login import LoginManager

import forms
import models

app = Flask(__name__)
app.secret_key = 'this is our super secret key. do not share it with anyone!'
login_manager = LoginManager()
login_manager.init_app(app)


@login_manager.user_loader
def load_user(userid):
    try:
        return models.User.select().where(
            models.User.id == int(userid)
        ).get()
    except models.DoesNotExist:
        return None


@app.before_request
def before_request():
    g.db = models.DATABASE
    g.db.connect()


@app.after_request
def after_request(response):
    g.db.close()
    return response

@app.route('/register', methods=('GET', 'POST'))
def register():
    form = forms.SignUpInForm()
    return render_template('register.html', form=form)

import datetime

from flask.ext.bcrypt import generate_password_hash
from flask.ext.login import UserMixin
from peewee import *

DATABASE = SqliteDatabase(':memory:')


class User(Model):
    email = CharField(unique=True)
    password = CharField(max_length=100)
    join_date = DateTimeField(default=datetime.datetime.now)
    bio = CharField(default='')

    class Meta:
        database = DATABASE

    @classmethod
    def new(cls, email, password):
        cls.create(
            email=email,
            password=generate_password_hash(password)
        )


def initialize():
    DATABASE.connect()
    DATABASE.create_tables([User], safe=True)
    DATABASE.close()

from flask_wtf import Form
from wtforms import StringField, PasswordField
from wtforms.validators import DataRequired, Email, Length


class SignUpForm(Form):
    email = StringField(validators=[DataRequired(), Email()])
    password = PasswordField(validators=[DataRequired(), Length(min=8)])