How to add html/comments to the discussion tab?

What is the proper way to add in some html/comments to the discussion tab? Is there a hook that can be used. Right now it is set to only owners can discuss. Which seems to work fine but those that do not have the item, see the Discussion tab with say 10 but when they go to it is just blank. I would like to put a message here, possible only for those that have not purchased it, that says “Only logged in users that have purchased this will be able to see and participate in discussions.” Something to that affect.

I am currently using CSS to hack some text in there but it is not the ideal way I am sure.

#tab-discussions::before{
    content:"Only logged in users that have purchased this will be able to see and participate in discussions.";
}