I've been pondering writing collectives/co-ops/groups for a while now--not critique groups, but organizations of writers who work together to promote their work.
I'm interested because I've seen how effective group promotions can be. Offering a Smashwords coupon by myself isn't nearly as effective as taking part in their annual e-book sale. That seems to be a major draw for something like the Indie Book Collective--I think that when I put Trang into KDP Select I will also look into taking part in something like that.
The other thing is that groups can do certain things that individuals aren't really allowed to do. For example, I took part in a Meetup group focused on e-book production and promotion. One of the people there went to a sci-fi con, hoping to pass out promotional material for her books, and felt very overwhelmed. Well, back in my Browncoat days, we'd get tables at sci-fi cons to spread the word about Serenity--if your organization isn't selling anything and you put in your request early enough, you can usually get the table for free. Hanging out at a table full of promotional material is presumably less intimidating for the writers, while said table full o' goodies is presumably more interesting to the readers.
So, using the Meetup group's name, I put in a request for a table at Foolscap, and they gave it to us. Immediately, the group stopped meeting (oops), but I'm hoping that people with books out are still interested in doing it. (In fact, if you write speculative fiction, live in the Puget Sound area, and want to participate, fill out that "Contact Me" form over to the left there. I'm encouraging people to do actual promotions--coupons, links to free fiction, whatevs--rather than just boring ads that say, "Read Me! I'm Awesome!")
We'll see how it goes, but you know, if doing a table works for the readers and works for the writers, it seems silly to not do it. There needs to be an organization, though--the cons would be a lot less willing to offer a free table to an individual writer.
Another thing that might be easier for a group to do is to sell to independent bookstores. Obviously this would not be easy, just easier--you'd have to do the Web site and flyers (which would be an up-front cost). But if it's a question of getting enough paper titles together to have a catalog, that's going to be a lot easier for a group of writers to do than an individual.
People talk about this sort of thing here--it does sound like a lot of these groups are very author-centric, with people just Tweeting about how awesome other group members are, which I don't think would be helpful. (And some of it is really just regular writing-buddy type stuff.)
In some cases they pitch in to buy ads together, which could be helpful. I suppose you and a writing pal in the same genre could decide to split the cost of an ad in Romantic Times or Locus or whatever. On the other hand, if it were an on-line ad, where would the link take you? There would need to be a Web site with links to both writers' e-books--so you're back to an organization....