Is it necessary that every user in a buyer group get's a role in the Q&A?

No. You can set up a buyer group and decide to only grant access to the Q&A tool for a selection of users in that group. 

Keep in mind that without a Q&A role, users can not see any questions or answers asked in the Q&A tool. Making it impossible for them to participate or collaborate. 

You can make a distinction between "Question Drafters" and "Question Submitters", or when you want to exclude users, "No Q&A". See the example screen below:

  Q&A Tip: If you want to change one of your user's Q&A role, visit Change user's role to learn how to perform this action.