If you're using TurboTax Online, unfortunately there is no way to add your spouse after you have started your return alone. The only option is to start a new return and answer “My spouse/partner and me” to the question “Let's get started. Who are you preparing this return for?”.
To create a new return, click on “Tools” in the black menu in the top right, and select “Return Manager”. You can create a new return by scrolling to the bottom of the list and clicking "Continue" next to "Start a new 2017 return".
If you're using the CD/Download version of TurboTax: