Starting a Group Blog series:
- Preparation - Strategy
- Pre-Launch - Expectations
- Launch - Execution
- Post-Launch - Growth / Conclusion
The first part of your strategy is the same as for starting an individual blog: choose your focus/niche, determine your expected audience, and choose the blog name / domain name (and trademark them if you choose). Once you have those, start compiling a list of related blogs, both as a resource for your writers (who can react to related posts) and to use in your promotion efforts.
Then start recruiting your bloggers, and determine (probably with their help) how you're going to structure ownership and copyrights for this blog. Most likely the bloggers will want to retain copyright on the posts they write, while the sponsoring organization will have the right to publish those posts indefinitely. If you're interested in rights to republish the content in other forms (ebooks, podcasts, white papers, books), make sure to discuss that early! See the content reuse and leaving the blog sections (in later posts in this series) for additional considerations. The sponsoring organization will probably own the blog name, domain name, and design.
You'll want a variety of bloggers with different expertise and viewpoints, so don't rely on one person's network to find them. Consider teaming up with established bloggers, and consider recruiting excellent writers who've never read a blog. Note that in a group blog you probably don't want ghostwriting, because that additional layer of coordination will be frustrating over time.
All your bloggers should have a full understanding of the focus/niche of this group blog. Unless it's intended to be a blog about your organization, don't force bloggers to write only about the sponsoring organization and its work: the blog will have far more credibility if its authors provide interesting, wide-ranging voices on a topic rather than sticking to the company line. It's actually good to be criticized occasionally on the blog (but do offer your point of view politely in the comments). Controversy attracts readers and starts discussions!
Plan your categories early. You'll want a way to see all posts by one author, but that shouldn't be your only category scheme. One of the best things about group blogs is the way different bloggers' posts play off each other, so plan to categorize posts by subject area as well. If you're planning way ahead, a nonobvious piece of advice is to think about categories as forming the basis of book chapters, so it will be easier to repurpose your content in the future.