models.py

from django.conf import settings
from django.db import models


class Project(models.Model):
    editors = models.ManyToManyField(
        settings.AUTH_USER_MODEL, through='EditorMap', related_name='editable')
    viewers = models.ManyToManyField(
        settings.AUTH_USER_MODEL, through='ViewerMap', related_name='viewable')


class EditorMap(models.Model):
    project = models.ForeignKey(Project)
    editor = models.ForeignKey(settings.AUTH_USER_MODEL)


class ViewerMap(models.Model):
    project = models.ForeignKey(Project)
    viewer = models.ForeignKey(settings.AUTH_USER_MODEL)