Once you’ve started to deploy more than a couple of HomeKit accessories in your home, you’ll likely reach a point where you’ll want to ensure that your other family members can also access and control them. Fortunately, Apple has considered this in the design of HomeKit, and in fact has even improved these features with iOS 10 and tvOS 10.
To add a user to your HomeKit home from your iPhone, simply open up the “Home” app and tap the arrow in the top-left corner. On this screen you’ll see the name of your home, a listing of any of your Home Hubs, and an “Invite…” button. Tapping this allows you to invite anybody with an Apple ID to share control of your home, and if you’re using iCloud Family Sharing, the app will even helpfully offer up your list of family members to get you started. Note that if you’re managing more than one Home, each one gets its own sharing list, so you can let the whole family control the lights in your main home, while perhaps only allowing your spouse or partner access to the cottage.
You can add additional users to share access to your HomeKit configuration even if you don’t have a Home Hub, however you’ll get the most out of the feature if you’re using a fourth-generation Apple TV or have an iPad running iOS 10 set up to act as a Home Hub. Without any kind of Home Hub, additional users will naturally only be able to control HomeKit accessories when they’re on the same Wi-Fi network, and while a third-generation Apple TV will provide basic remote control capabilities, you’ll need a fourth-gen Apple TV with tvOS 10 or an iPad set up as a Home Hub to take advantage of more granular permissions, such as granting access for family members to edit HomeKit devices, or disabling remote access.
Note that you can also still invite users even if they’re not running iOS 10; users on earlier iOS versions won’t be able to use the Apple Home app, of course, but they can install and use any one of the various third-party HomeKit apps, or simply control HomeKit accessories using Siri commands without the need to install anything else at all, provided of course their device supports Siri in the first place.