First off, I'm planning on running my project on google app engine so I'm using djangoappengine which as far as I know doesn't support django's ManyToManyField type. Because of this I've setup my models like this:

from django.db import models
from django.contrib.auth.models import User

class Group(models.Model):
    name = models.CharField(max_length=200)

class UserGroup(models.Model):
    user = models.ForeignKey(User)
    group = models.ForeignKey(Group)

On a page I have a form field where people can enter a group name. I want the results from this form field to create a UserGroup object for the user - group combination and if the group doesn't yet exist create a new Group object. At first I started putting this logic in the UserGroup class with a add_group method but quickly realized that it doesn't really make sense to put this in the UserGroup class. What would the proper way of doing this be? I saw some stuff about model managers. Is this what those are for?