Book marketing is a fact of every author's life, whether indie or traditionally published. Work as hard as you want, you still can't reach every single reader who might like to buy your books. If you can only reach a certain percentage of the reading population, it stands to reason that your fellow authors have the same problem. Why not combine forces and double or triple your reach? Collaborating with your colleagues is a great way to reach potential readers you might otherwise never have found, and it won't take much more effort than you're already making. 
The Competitor Problem
The authors that object to this plan look at book promotion/sales as a zero-sum game. In this view, if a reader picks up someone's book, they won't pick up yours instead. Most of us know this isn't true, though. Readers will justify a way to buy as many books as they want to read. And if they find another one that looks good after that, they'll buy it, too. Having promotions that include more than one author just means more potential readers will be interested in following it.
Finding Teammates
Your promotion "teammates" are probably authors who you see online every day. Look through your writing groups and social media for authors who write in your same niche. You want to promote to the same type of reader, so it makes sense to choose similar authors for your marketing group. After all, romance readers aren't avidly looking for science fiction novels, not on a regular basis, anyway. Lay out your plan to half a dozen authors or so, and try to have at least three in the group when it finalizes.
Newsletters
All authors in the group should already have an ongoing newsletter with a similar fan base. Decide how often you're going to promote each other's work. You can add links in every issue, post news about each other once a month, or make a separate newsletter that goes out to all readers that includes all of your input. Whatever you choose, make sure it's something everyone in the group can and will sustain.
Social Media
Present yourselves online as a united front. The more often your respective fans hear about the others in the group, the more they'll connect your names with books they love to read. Cross-promote each other's new releases, have online parties with giveaways and Ask the Author sessions, retweet or share everything else book related that each of the members posts.
Back Matter
The back matter in eBooks is crucial real estate, reserved for promoting the sales of your next books and signup for your email newsletter. Add the same information about other members in the group in each other's back matter. The increase in eyes on your information will be significant, and it should benefit every group member equally. Putting in a blurb about someone else's book or having a signup for your newsletter in another author's work also adds interest to the back of each book, making it more likely that readers will keep turning the pages until the end.
Keep the Faith and May the Force be with You!
