If you have the same booking number (PNR) and provide all the passengers in one auto check-in request, the airline will provide the seats near each other. So you will be sitting together (that rule doesn't work with low-cost airlines, that request extra money for that).
If you want to add passengers for auto check-in, tap the "+" button before hitting "Submit" and fill in their information and preferences. Then, submit your request.
By default, (based on availability) the traveling party would be seated together.