DH used to work for an airline and we flew standby all the time before we had kids. His sister works for American and we flew on her buddy passes quite a bit. IDK if Southwest works the same but DH's airline and Amercian worked on seniority. So if there was a waiting list, paying passengers who missed a flight or something were at the top of the list, then buddy passes by seniority. So if there were 5 buddy passes on the wait list already when we checked in, DH's sister's 28 yr seniority would almost always be higher than the other buddy passes seniority and we would bump the lowest seniority off the plane if there wasn't room for everyone.
Even with all that said, it's a crapshoot b/c you never know how many paying passengers are on the standby list that will be boarded before you and you don't know how many other buddy passes are waitlisted for that flight too. Oh, and we always tried to take the first flight out in the morning b/c previous flight buddy passes that didn't make it on that flight were moved to the top of the buddy pass standby list for the next flight. There were times that we would sit at the airport for 15-16 hrs before actually getting on a flight, even when the flight we originally tried to get on looked pretty good (quite a few seats open) when we checked before we left for the airport. I've always been amazed at how quickly things can change with these flights. Oh, we even gotten our boarding passes and been on board the flight and been called off the flight b/c a paying passenger showed up and they needed to get them on the flight, so we were bumped.
Free is great but what you actually deal with to fly free is a pain. If you can swing buying tickets, do yourself a favor and buy tickets.
As far as getting 5 people on the same flight together, it could happen but I wouldn't hold your breathe - the more in the party the longer you'll be waiting. You may very well have to split up if you want to get to your destination in a timely manner. Have a back up plan and be prepared to sleep at the airport. If all 5 people must fly together, I would buy tickets.
Good luck!