Does every user in a buyer group need to have access to the Q&A?

No. You can set up a buyer group and decide to only grant Q&A access to a selection of users. You can also make a distinction between Question Drafters and Question Submitters. See 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.