Nearly every pug I play in either someone else or if not myself will explain it.
Mainly ground combat is where people show up desperately unprepared and quit the map.
But I will say part of the charm of pug's is also the adventure, sometimes you play with a team that gets the optional like a walk in the park, other times I am carrying the remaining 2 people who didn't ragequit and have to use google translator to explain the map to them in german.
One engi I played went from dying with turrets and duel pistol to mines, shotgun and c4 killing everything like mad crazy frontline so much I got combat high while fighting on a map I played over and over.
However it has to be said I have everything and enjoy the fighting in itself so it' easier for me to stay calm when things go bad.
The thing is, as long as the person has even half-decent gear, all that's really needed is a little attention to teamchat. That's it.
If you go into an STF, and ask, "What do you want me to do?", you're probably going to have someone tell you, "Go sit on the right side and kill the probes before they get to that swirly timegate," or maybe "Just follow us around and pop these probes, but don't shoot the Cubes until we do."
It's all about communication, IMHO. If you're new, tell your team, so they know to inform you; if you have a question, ask. It's that simple.