Because SW does not assign seats when you book, you get assigned a section, A, B, or C, when you checkin online. When it comes time to board, those who qualify as 'preboarders' get to board and choose their seats. Then, those who got section A boarding passes board the plane and choose their seats, then section B and finally section C people. If flying with a group that wants to sit together, then you should checkin at the 24 hr mark. I've never had an issue being seated with my flying companions so far, and even those with the tail end of the section B's seem to be able to find seats together, but at the back of the plane.
As far as SW actually participating with DME...all airlines participate upon arrival at MCO. It's the end of the trip and the return to the airport where 'participating' comes into play. Those that are flying a 'participating' airline are entitled to check their luggage at their resort, and get their boarding passes right then. When they take the DME bus back to MCO, they have only their carryon bags and can go right to security. With SW, you can get your boarding pass printed out at the front desk, but you are not able to check your luggage at the resort. There is a special spot for those that are able to do that....not at the front desk per se.
I hope this doesn't just confuse matters for you. If so, just tell us what you find confusing and we'll try to explain.
