I got married in 2016 and I want to transfer my spouse's 2015 tax return information into our joint 2016 filing, which we've started in my account. She used TurboTax to file her 2015 taxes. How do I do this?

To begin a new joint return on TurboTax:

You will not be able to merge two previous TT account to start your new joint return.  You can import ONE of your 2015 returns into a new return, so choose the most complicated one.  The other spouse’s information needs to be entered manually.