If you have the same reservation number (PNR) and you indicate all passengers in one automatic check-in request, the airline will assign seats next to each other. This way you will be seated together (this rule does not work on budget airlines, which charge extra money for this).
If you want to add passengers for automatic check-in, click the "+" button before you click "Submit" and fill in their information and preferences. Then submit the request.
By default, (depending on seat availability), travelers will sit together.