It’s an app with a gray gear, usually found on the home screen. It will be grouped with your other apps, in the bottom half of the page. It will turn green. Now, Snapchat has access to all of your phone contacts.

TL;DR in Chinese:
打开iPhone的“设置”。 点击Snapchat。 将“通讯录”按钮滑动到“打开”位置。