Assuming a group blog is a partnership (re: two or more persons sharing ad revenue), then a good planner might advise that co-bloggers not to share - unless the blog is making a lot of money. Sharing the money might mean there is a general partnership. Thus, to avoid the problems presented below, they'd have to create an LLC (filing in Delaware, of course). Unless you have a form Operating Agreement, you would have to pay someone to write it, or spend time finding one. (Thankfully, my Business Planning prof made us draft all the necessary documents as part of our class grade). Then, at tax time, the bloggers would be required to fill out an informational filing form. And on and on.
Who knew that putting ads up could raise so many issues? Anyhow, allocating ad revenue might not be worth the costs - again, unless we're talking about a lot of dough. I suspect that David Giacalone is smiling.