Should I add my stepdaughter to my information if we are filing joint, or will we be going in his account to do his and leave her there? I always claim our other 3.

If you are filing a joint return then you are filing ONE tax return for the two people who are married to each other, and ALL of your information goes on that ONE tax return, including income, dependents, credits, deductions, etc.  There is no using two accounts to prepare the return.  So if you and your spouse are the custodial parents of your stepchild then she goes on your joint return as your dependent.

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 transfer ONE of your 2018 returns into a new return, so choose the most complicated one.  The other spouse’s information needs to be entered manually.

When you enter your own information in Personal Info, you have to answer the question "Were you married?"  If you click the button for Married, then a drop down will appear that asks, "Do you want to file this return together with your spouse?"  Then you choose yes to file a joint return.  You would then enter your spouse's information into Personal Info.  Whenever you are entering income information there should then be a spot for you and for your spouse's income information.