Transferring number between 2 family plans?

Sure you can do it by calling 611 customer service or  att.com/transferofservice

http://www.wireless.att.com/answer-center/main.jsp?solutionId=KB95468&t=solutionTab

Where can I get the form to start a Transfer of Billing Responsibility (ToBR) online?

How do I Transfer my service to another person online? Can I request a Transfer of Billing Responsibility online at myWireless Account? Answer:
To initiate a Transfer of Billing Responsibility (ToBR) online you will need to log in to your myWireless Account.

1. Select Support  located near the top of the page.
2. Select the More Phone/Device Support link located in the Phone/Device Support  section.
3. Select the Transfer Your Phone link located in the Most Popular  box at the bottom of the Phone/Device Support  page. If you have an iPhone device, or the link does not appear on the page, please log in and go directly to Transfer of Billing Responsibility Request.   
4. Follow the instructions on the Transfer of Billing Responsibility Request page to complete the form.
5. Read the Transfer of Liability information; if you agree select Accept & Check Eligibility.

You will be presented with a confirmation page and an e-mail will be sent to your e-mail address on file.  
 

Note: This authorization only remains active for 14 days from the time the transfer is initiated. After 14 days, the authorization will become inactive. Please be sure to remind the recipient of your account to acknowledge and complete the transfer of service as soon as possible.