If you start a new return without transferring any info from last year, it will ask you if you want to prepare your returns together. If you transferred your previous year’s return that you prepared only for one spouse, it will ask you the question after you have transferred your info.
If you have started a return for only yourself, unfortunately, there is no way to add your wife in TurboTax Online. You would need to create a new return to prepare a return with your spouse, which you can do by selecting “Start a New Return” from under “My Returns”. Once you have started your new return, you can select “View All Tax Returns” from under “My Returns”, and delete the unwanted return by clicking the small Trash Can next to it (or “Delete” underneath it, on mobile).
Please note that when you prepare your return with your spouse, this counts as two returns, and you will be charged the same price for both returns.