When you're logged in, click "view your Habbo home", then proceed to look for the "edit" button, once found click that then click on widgets, a popup will show with a list of available widgets (e.g., guest book, rooms, friends list), select the groups widget and place it on your HabboHome, you will then see the groups you've joined...

If you want, for example, to set Habbox's group as your favourite, look for Habbox and click the green +, then click add to favourites, you will then have the group badge.