In order to add members to a client account, it must first have an Account owner and it must be an adult. Once you've added another adult member to the account, you can change the Account owner, if needed.
To create a new member (of any age), access the Account members subtab by clicking on the desired client's account.
In the Actions menu, click + Add a person and fill out the form.
You can add as many members as needed. The account owner is responsible for all the added members.
Once a purchase is completed, the account will get updated with the information provided on the form.