Family plans allow for two parent accounts. The initial parent account will have been automatically created using the email address you used to subscribe with. Details on how to log in will be sent to you in your subscription confirmation email.
To add a second parent, log-in to your own parental account, then go to Manage and click Add Parent.
Once you've fill in the required details, a short email will be sent to their email address with a link for them to set their password.
Log in to your parental account (see above on how to do that), then go to Manage and click Add Child.
Please note that only three child accounts are available on a Family Subscription.
Log in to your parent account, then go to Manage to view your family's accounts. Tap the bar for the parent or child and press Edit User. Change their details, followed by the Save button, to edit them, or press the red Remove link to delete them.