It can take a bit of messing around to find the right widget and type. The easiest way to do that is to look through the source code. Searching for applyDecorators in the widget files lets you know what types are available. The following works, but you should test it. Do a console.log on the attrs variable to see what is available to you. It will take a bit of CSS to get it to look good.
<script type="text/discourse-plugin" version="0.8.25">
api.decorateWidget('post-contents:after-cooked', helper => {
let h = helper.h,
attrs = helper.attrs;
if (attrs.primary_group_name === 'your_primary_group_name') {
return h('div.group-signature', h('a.group-user-link',
{ href: '/u/' + helper.attrs.username }, 'See my profile!'));
}
});
</script>