You could still create groups as before, with Ctl + G. This will create a group with a single visible collection. However you can also create a group from an existing collection. Just go to the menu you get in the outliner when clicking in a collection and pick "Create Group". Remember to instance the group afterwards, or link it into a new scene or file. The group and the collection are not kept in sync afterwards. You need to manually edit the group for further changes.