Another benefit to App Groups is the ability to share a NSUserDefaults database. This also works for App Extensions (notification center widgets, custom keyboards, etc).
Initialize your NSUserDefaults object like this in all applications in the app group and they will share the database:
Objective-C:
[[NSUserDefaults alloc] initWithSuiteName:@"<group identifier>"];
Swift:
NSUserDefaults(suiteName: "<group identifier>")
Keep in mind everything from the [NSUserDefaults standardUserDefaults] database for each application will not carry over into this database.
The documentation gives a correct example as well (As of Beta 3).
And don’t forget to synchronize the database:
[yourDefaults synchronize];