Having a shared list that you and your significant other (or whoever) can see and add to is pretty awesome. Gotta buy lightbulbs? Throw that on there, and whoever goes to the store first can pick 'em up. This is great for a list of chores that need to be done, too, or event planning with your pals. And if you're using iCloud, it's got this functionality baked right in as of OS X 10.8.2. Sweet! So to configure a shared list, first open the Reminders program on your Mac.
Then hover over any of your existing iCloud lists and click on the radio-wave icon that appears.
You can also click the plus button at the bottom of the Reminders window to start a new list to share instead.
Anyway, after you click that radio-wave icon, you'll get a pop-up where you can enter your contacts' names or email addresses. Make sure you're sending this invitation to their proper Apple IDs, though.
Afterward, the person or people you're sharing with will get an email like this:
Once they accept, you're all done! Everyone will see items added to the list on any device syncing Reminders from iCloud. Then you can be sure that your grocery list is always up to date, especially for those critical items you need on hand.
What? Don't you judge me.