How to share entire address book?
You can set up multiple different iCloud accounts in the "Mail, Contacts, and Calendars" preference panel, and also on your iPhone and iPads.

I have accomplished a shared address book with my partner by each of us having a primary iCloud account (me@us.com, her@us.com) - and then a shared iCould account us@us.com.

One annoying downside is that the Lion Address Book app labels both address books as "iCloud" making it a bit hard to distinguish what contacts are where.  We solve this by having different contact lists in each address book, so they are distinct.

It seems with just a little bit of work Apple could make this MUCH better, but this is the best solution I have seen so far.