They need to go to the Edit Profile page and select the group badge they want displayed:
@Giorgio-Chiodi 's thread only displays when you click on it from the category, if you link directly to it, you get a this page redirects in a way that it will never complete.
To reprocude, go into NodeBB development category, click giorgios thread of ..., then refresh the page. Doesn't do it with any other topics. Seems that the elipsis in the topic title is causing a few issues.