[Answered ]-How to define user and group being unique together in django

The answer is in your question: unique_together.

Here is a reference to the official documentation:
https://docs.djangoproject.com/en/1.11/ref/models/options/#django.db.models.Options.unique_together

Example:

unique_together('user', 'group)

Assuming that user and group are referencing to other models, this should be set in the model (db table) that connects both models, conventionally called something like yourapp_user_group.