The Add Contact to a List action allows you to add a contact to a specified list when that contact enters the Journey's funnel. See Fig 1 below. 

Fig 1

To select which list to add the contact to, click the Add Contact to a List action node once you have added it to the Journey work area. Doing so will bring up this popup ("Properties") on the right-hand side of the screen (See Fig 2 below):  

Fig 2

Clicking the "List" input field below the "Add Contact to a List" label will bring up a vertical menu of all your available lists from which you may choose the target list. (See Fig 3 below). Select the list by clicking its name.

Fig 3

Once you have finished with the email setup, click the "Save Properties" button.