Only users with "Post" access to Q&A can communicate with the Q&A coordinator directly. In case other users should be limited to interactions within their group only, please provide them with "Post to own group" access.
Levels of access to Q&A can be assigned individually on a user level, instead of a group level.
To change users' access to the Q&A section, please follow the steps below:
Navigate to the Users tab;
Select the users (tick the check-box next to these users) who should be limited to communication within their group;
Right-click on any of the selected users and choose "Change access to Q&A" => "Post to own group".
Do the same to grant "Post" access to Q&A to users who should be able to post questions to Q&A coordinators as well as their group mates.