Hey abigail2,
Thanks for you post. Firstly the previous accountant will need to transfer the billing to the client if this has not already been done. Then, your client will need to subscribe initially with their own payment details to reactivate the account (they will be prompted to subscribe when signing in). Once this is done, they can add you as their new Accountant (Gear icon > manage users > accountant).
Once you have accepted the accountant email invitation you will see them listed in your client list in QuickBooks. To move them to your billing, go to the Gear icon > Your account > Your subscriptions > Add existing clients and the client will appear on this list. You will then see the current offering which is dependent on their current subscription type (SimpleStart, Essentials, Plus).
Thanks!