Here's a link to the App Store.
Here's the blurb:
Find My Friends allows you to easily location your friends and family from your iPhone, iPad, or iPod touch. Install this free app on your iOS 5 device and sign in with the Apple ID you use with iCloud. Adding a friend is easy - just send a request to see their location. Once your friend accepts using the Find My Friends app, you will then be able to see that friend's location on a map. You can also choose to share your location for a limited period of time with a group of friends. Use Find My Friends to keep track of your traveling companions when you're on a vacation. Or to see if the kids are home from school. Or to find the friends you're meeting for dinner.
You will need an iPhone, iPad or iPod touch running iOS 5 to make use of this.
Here are some pics: