I believe that it is $10 pp per route. You can select to do it for only one way (i.e. the return flight.) My family and I usually end up flying SW. I check my parents in all the time. The key is to literally check in at EXACTLY the 24 hour mark. Like clear your schedule, be home, have your check-in information typed into the fields and when the clock changes, hit the button.
In this method, we have ALWAYS gotten A seating. One time, flying from Phoenix (SW hub), I got the very LAST A boarding. LOL. I checked my parents in today, as a matter of fact, and they got A22 and A23.
SW used to reserve A1-15 for business class passengers. It used to be that there would be a big gap in the seating lines, because so few passengers paid for the business class. Now they offer early bird check-in. I don't know if this is in addition to, or replaces, the business class. If you pay for early-bird check in, then SW will automatically check you in, and they will do it BEFORE everyone else can at the 24 mark.
Generally, when you consider the capacity of a typical SW plane, 149 seats, usually 6 seats across, with one third of the seats being aisles, windows, and middles. This means that there are roughly 47 aisle seats and 47 window seats and 47 middle seats. There are 60 people in each boarding group. You know the size of your group and what seating arrangement would work out best, but odds are, if you make checking in a priority, you should be able to find seats together no problem.