Friend group data model

[stefpiatek]

1. Fork the friend group repository to one of your accounts.
2. Add everyone else in your group as a collaborator to the forked repository.
3. Clone your fork locally.
4. Create a new branch named with a combination of your team
   e.g., dpshelio-ageorgou-stefpiatek.
5. Write your code in the file group.py to do what the exercise asks - see the instructions in the README file of the exercise repository.
6. Commit your changes to your branch! (with a meaningful message)
7. Push your changes from your computer to your fork.
8. Create a pull request (PR) from your branch to my repository.
   Add a meaningful title to that PR and don't forget to mention your partners in the description (as @username) and a link to this issue: Answers UCL-RITS/rse-classwork-2020#8