I've done plenty of searching and I can't find ANY information about having multiple developers working on a Chrome extension.
Basically, I just want to grant another developer access to manage my Chrome extension in the Chrome Developers Dashboard. (ie so that it is listed in their Chrome developer Dashboard and they can publish it/change the logo etc).
EDIT 6 Aug 2013 - The Group Publishing feature allows organisations to create a group such that the extensions can be managed by multiple developers. For more info, see Group Publishing.
Old answer
An extension in the Chrome Web Store can be managed by only one Google account.
Assets (screenshots) are usually not updated very often. On the other hand, it's common to update the extension's source code and description, so letting the main developer manage the extension listing makes sense.