Apple Arcade subscriptions allow access for up to "six family members." Subscribers will need to set up a Family Sharing account to be able to share their Apple Arcade subscription with up to five other accounts that have the same credit card associated with their Apple IDs.
You can set up a Family Sharing account by going to Settings > [your name] > Set Up Family Sharing. All you need is an active Apple ID signed in to iCloud and iTunes to create an account. Your Apple ID can only be part of one family at a time.