How about Dropbox? Everyone in the group installs the app, you upload it to Dropbox and share a link, allowing everyone else to download the book. You don't need to share a Dropbox account, if you upload to your account and share a link, anyone with the link can download the file.
The one issue might be DRM. If the ebook is protected I think you will need to share the iTunes account. In case that's geek speak to you, this means that if you bought the book on iTunes, it must be downloaded from your iTunes account. If you emailed a copy to someone else, they would not be able to read it. Also realize that iPads are registered to iTunes accounts, so others won't want to switch to yours as it would update their iPad in unwanted ways!
If the ebook is not copy protected, Dropbox would work fine, or you could just email it to everyone. If your ebooks are copy protected, then the protection is designed to prevent you sharing it! iTunes may allow lending, I have a Barnes & Noble nook which has a "lend me" feature, but it lets you lend the ebook to only one person at a time.
Patrick