There isn't much Google+ 3rd-party integration yet because Google hasn't made the API available to the public. They do have a invite-only developer test going on soon However yesterday I saw that Google actually just bought a small start-up called Fridge. That company had a lot of interesting features that are now likely to be integrated into Google+.
Features from Fridge's homepage:
- Create a group to share with friends & family
- Organize events, outings and parties
- Plan and coordinate group vacations
- Manage classmates and study groups
- Collaborate with co-workers and team mates
In the mean time, depending on the size of the group I guess, you could perhaps use Google Doc's collections to manage your topics, sharing and commenting.