So when you share a "group discussion", the big social websites detect the author's avatar as the thumbnail using open graph. I'm trying to inclue the right tags so it grabs the image from the discussion itself.

I'm thinking in template/default//groups.viewdiscussions.php to include along the lines:

<meta property="og:description" content="<?php echo $discussion->message; ?>" />
<meta property="og:image" content="<?php echo $discussion->message; ?>" />

Has anyone else tried this?

Hi, Bryan.

$discussion->message; doesn't contain authors avatar.
Additionally just adding <meta property="og:image" content="<?php echo $discussion->message; ?>" /> will not work as it will be displayed in BODY of page... so it will be ignored. Meta tags need to be displayed in HEAD of the document.
And on discussion pages there are already meta tags like this... so if you add them in head (using some php code) they will be duplicated so robots may consider it as a spam or code mistake...

To achieve what you need in here you'll have to hack core files to display other values for meta tags on discussions pages. Core hacks are not recommended nor supported as they may not working with future JomSocial releases and/or cause issues that will be not supported. Users with core files hack refuse to upgrade their installation to keep those hacks and they are stuck with outdated and unsupported versions.

So only what I can recommend here is to ask in Community (the best after 1 January) if JomSocial 4.0 will have this enhanced or post a feature request here: uservoice.jomsocial.com/forums/101561-feature-request