It seems to me that the groups icon is the way to communicate security settings - the one that is included on the category buttons in various places for when categories have non-default security. This would be hidden everywhere on public groups as it is already. When there are non-default security settings it would show up and be clickable to show a popup with info about groups and security settings. This would be identical to what you see on the security settings page, except that on the popup the group name would be clickable to take you to the group members list.
Here’s a very quick and dirty mockup of category page:
I don’t know where the group icon would live on category or topics pages - there is so much going on in those pages that is beyond me. 
maybe the best place would be below the WATCHING button with “SECURITY” or “GROUPS” heading, with similar functioning as WATCHING. Or it could go at the top next to the star, or next to the topic date, or within the topic summary box. f it goes into the topic summary box the full contents of the popup could also be displayed when that summary box is opened up to show the details.