[RPG] Introducing System Mechanics to new players

By complete and utter coincidence, combined with a little bit of incompetence in leadership, the new players show up at an opportune time. As it happens, there were two groups assigned to do the current party's job, but nobody realized it until both groups had been sent out already.

The first group is the current party, and was assigned to this task by (the king? his spymaster? his top general? his other top general? the captain of the palace guard? a scheming duke?) The second group consists of the new players, who were assigned to this task by somebody entirely different. (If you want to be complicated, have the second group be there for a different reason. Make up another reason for them to be there. The players haven't explored the entire cave system yet, so you can still add something in an area they haven't seen.) If you think that "just the new players" is too small of an adventuring party to take on this cave system, then the new players' party was once larger, but some people got killed along the way.

As for how to dispose of the characters that left the game: adventuring is dangerous. People die, sometimes with no warning beyond "Hey, what's that funny soun---". Death doesn't respect character back-stories or plot arcs or convenience; if somebody suddenly has a gaping hole in their larynx, being in the center of a web of plot won't help them. Of course, if your players have easy access to resurrection, then that won't fly. So, if you truly need a way to remove the PCs without killing them, then the second group could have orders for the PCs who are leaving, that are more important than their current task. Or an offhand comment by one of the newcomers makes one or more of the PCs suddenly run off to rescue a loved one, who was living in a city that just got besieged by an invading army.

Note, if you kill the extraneous PCs, and you make it a "deus ex machina" moment, make it clear to your remaining players that you're not going to kill them that way while they're still playing. Most players don't find it at all fun when the GM randomly kills off PCs; don't let your players pick up the idea that their PCs may suddenly be taken from them if they annoy you.

Having recently played a Pathfinder one-shot where half of the players were completely new to the system; many of them were confused, including our normal D&D 4E DM. Character creation alone took around an hour and a half due to the wide variety of options. Despite this, once we actually started playing things rolled along decently quickly. Based on this experience, I can certainly recommend a few things to avoid.

Avoid too Many Options

Option paralysis is a very real thing even for experienced players; when someone is just starting out with Pathfinder, they lack the ability to make an assessment of important skills, feats, and variant classes or alternate class features available. I'd recommend letting the players in question choose their race and class and whipping up a set of recommended Feats and Skills they should take to prevent this paralysis; keep the character building aspect simple at first. If you had a lot of time on your hands, you might even make two or three of these recommendation sheets per character and let the player choose between them; this would serve as a simplified introduction to feats and skills. You can do something similar with spells for spellcasters. It's worth stating that these options are important to Pathfinder; it's just important to introduce them slowly. If you wanted to go even further, you could recommended against prepared spellcasters and make them keep to spontaneous spellcasters to keep their options low. I would also recommend against Summon type spells, since they usually require the player to have a detailed knowledge of all the creatures on the Summon List.

Keep it Low Level

No, lower level than even that. I'd recommend starting around level 1 or 2; higher than that, and the feat and spell choices multiply quickly. We all played as Level 4 Monk variants in our one-shot, and feat selection alone took the bulk of our character creation time.

Make sure your Players know what they can do In Play

Depending on familiar your players are with other editions of D&D, many of these concepts might not need to be explained. Other than the obvious attack and damage rolls, make sure fighter-type classes know what their feats actually do; give them a sheet with the descriptions of their feats on it. You should be able to make sure your players understand the action system as well. With spellcasters, I would recommend printing off a sheet of their prepared spells to let them know what options they have immediately available to them. For all players, make sure they know what their class abilities are; if their options are described on their character sheets and can be quickly refereed to, they're more likely to learn them. Readily available descriptions for Feats, Spells and Class Abilities will reduce confusion at the table

All of these are pointing to one thing: Reduce Complexity. Your players will understand Pathfinder better when they aren't immediately thrown into the deep end of interlocking mechanics, build choices, and combat options. If you present options as they come up (or let players read and discover for themselves between sessions), your players will be less overwhelmed and have more fun.