Adding existing customer to family plan

There is a $19 fee to do a transfer of billing responsibility. 

As for how much your new plan will cost with two lines instead of two separate accounts with one line, we can't tell you that without knowing what plan you are on and what you're paying now. 

But generally both of you would pay a little less than you do for two individual accounts. 

You will have to do a transfer of billing responsibility. One of you can begin the process and the other one except the process. If you are on completely different plans, you'll want to decide which plan you are keeping and that person is the person who will accept the transfer of the other person's phone line. 

Accessories have to be paid off, but phone numbers and any installments will go with the transfer. 

https://www.att.com/support/article/wireless/KM1045265/

We both have the same plan which is about $110 per line

That is not an answer 

Login to your account online. The name of your plan is on the overview page 

Do either of you know what you are paying $110 a month for? 

Reading your bill might be illuminating 

It’s the unlimited elite plan

It doesn’t really matter.  You need to do a transfer of billing responsibilities (TOBR) as has been previously stated to accomplish what you want to do.

I'm assuming you want to keep your HBO benefit. If you assume responsibility you can put the other person on the premium plan which is cheaper, and offers more hotspot data 

The price for each line will drop $10 because you will have two line pricing. 

If you have not already signed up for some sort of discount program, like as a student, or AARP, , that would reduce the cost of the premium or Elite plans by another $10 

That is stackable with the auto pay and paperless billing discount 

This is the price for two lines, before your $2.50 price hike.

And also before the 99 cent price hike on the premium plan

Hey there @Tbmkellierae,

Thank you for reaching out to us. We are here to help and provide all the necessary information you're seeking.

In regard to transferring an existing customer from one AT&T account to another AT&T account, 

Here is how to transfer billing responsibility for a wireless number from one AT&T account owner to another. You can start the transfer online, and the future account owner can accept billing responsibility online. To begin the process follow these steps: 

1. Go to att.com/tobr.
2. Select Start transfer.
3. Read the tips and then select Continue.
4. Enter your ID and password.
5. Follow the prompts.

Keep in mind: 

• You can transfer one line or multiple lines on the same account.
• You must pay off or transfer any device installment plans with the lines subject to a transfer request. Learn how to pay off an installment plan
• You must pay off any accessory installment plans associated with wireless lines you want to transfer.
• You'll lose any voicemails associated with your lines when the transfer is complete.

Once you have completed the Transfer request to merge your wireless lines on your account. You will then accept the billing responsibility. As the new owner of the account, you must meet these requirements:

• Be at least 18 years old
• Complete a credit evaluation when accepting the transfer (You may be required to pay a deposit.)
• Accept the remaining contract term from the date of the transfer (if applicable)

You'll receive one final prorated bill, which covers charges up to the date the transfer of billing was completed. Visit our Billing Responsibility help page for additional FAQ when merging two wireless accounts.

-In regards to AT&T HBO Max subscription, as long as you don't change your plan that you're currently on i.e. Unlimited Elite, you'll have your access for free to HBO Max.

-Once the plan is changed it cannot be added back, as the Unlimited Elite plan is an expired plan which had HBO MAX subscription for free. 

-The current Unlimited plans do not have HBO Max free on them.

-So we'd suggest you to hold on to the current plan you have to access your Max for free.

Let us know if you have any further questions. Thank you for contacting AT&T Community & Forums.

David, AT&T Community Specialist.