Calendar sharing with iCloud and MobileMe
This is a dilemma that I too have been struggling with and am sorry to say that at this time, I don't believe a real solution exists.  The new sharing interface in iCloud seems to have removed any and all methods of sharing an editable calendar with a non iCloud user.  The only sharing option for non iCloud users that seems to have remained intact is the ability to create a public calendar, viewable from a web browser.  There is however a partial work around that will solve a portion of your problem, but not all of it.  

You can get around the new iCloud sharing interface and restrictions by moving the calendars in question from the cloud to each individuals respective Mac locally.  The simplest way I've found to do this is to export said calendar to your desktop and then open the resulting ".ics" file into iCal.  When prompted for a destination calendar, select "New Calendar" from the bottom of the list.  This will create a new calendar stored locally on the Mac with the same name and events of the first calendar. (Be aware, that this will result in duplicate events unless you choose to then delete the iCloud counterpart.)  Once you've done this, the new local calendar will give you a "Publish" option inside of it's contextual menu (Right or Ctl Click) which leads you to an option to publish to Mobile Me, giving you a web cal link that any standard calendar service can subscribe to.  

There are too major downsides to this, the first being that it only provides viewing options, no editing ability.  The second problem is that it is now dependent on syncing your iPhone to your computer in order to keep everything up to date.  Local Calendars are not privy to either iCloud or mobile me cloud syncing.  

I know that this is definitely not the solution you're looking for, but hopefully it will provided some sort of stop gap until either Apple or some of our more industrious developer friends out there come up with something both more elegant and fully functional.  As always, I highly recommence backing up iCal before moving and or removing any calendars.   

I look forward to seeing what the rest of the community comes up with as well, hopefully there is something simple that I have missed.