Hello,
You'll need to add a field in your form that stores an email address. Then, in the email data connection wizard, instead of typing an email address, click the fx button next to the "To:" field. Click Insert Field or Group button and select the email address field from your main data connection. So, the form sends email to whatever user types into the email address field.
Also, if you're running SharePoint, the easiest way to do this is by adding a Person/Group Picker control in your form. Right click on the control and select properties. In the SharePoint server tab, enter your SharePoint server URL. Then, modify your email data connection to use the DisplayName field which you can find under the Person/Group Picker node.
Hope that helps!