You can follow the steps here below to get to the section to Add a dependent
Log into your account, click orange Take me to my return button
Type personal information in the Search box
Then click jump to personal information
You should now see page that says Personal info summary
Scroll down and find the +Add a dependent button there and click it.
This will allow you to go through and add another dependent.
The other option would be to log into return by clicking the Take me to my return button, click the Personal Info (next to Federal Taxes tab) tab at the top and scroll down to find Add a dependent.