I need to add another tax return to my taxes

Just to clarify, are you trying to add another W2 to your tax return?

To add another W-2:

• From the Federal menu in TurboTax Online, click Wages & Income
• Expand the menu for Wages and Salaries
• Click Start/Revisit next to Wages and Salaries (W-2)
• Click Add a W-2
• Enter the information as shown on your W-2 and click Continue

If you have already submitted your tax return, you will not be able to make any changes unless the IRS rejects it. You will receive an email informing you that your return has been accepted or rejected by the IRS. 

No, I do my own taxes and my mother's taxes.  I need to add my adult daughter's taxes.  How do I do that.

Each tax return you prepare in TurboTax requires a separate account with a separate username and password, however, you can use the same email address for all of them. Be careful not to start another return by going back and overwriting everything in the return you just filed. This will replace your information and you won't be able to retrieve your tax return later.
For more information, please see the below TurboTax FAQ:

How do I start another return in TurboTax Online?

@marion0709