Dead and Back Again
If you've followed my advice regarding the agency theory of fun, then character death is bound to happen sooner or later. As a gamemaster, I believe that when character death occurs, it's your responsibility to introduce the player's new character into the game as quickly and seamlessly as possible. In D&D, the classic way to handle this is to introduce the new character as a prisoner found left for dead in one of the next few rooms (often with a suspiciously similar class and name ... ). It's quick and it works.
But sometimes, when the surviving characters find the new player character, they decide not to let him into the party. What do you do as GM now? It will totally depend on the social dynamic of your campaign.
In a Collective campaign, you need to remind the players that the new character should expect to be admitted readily and probably given some gear and assistance to get into play swiftly. In most Collective campaigns, the players have created heroic, good-intentioned characters, so this is not too much of a stretch. But even if their characters are world-weary cynics with deep suspicions about the world, in a Collective game the players agreed to put the group ahead of the individual, and agreed that each player is entitled to participate, so the need to introduce and integrate their fellow player should trump the particulars of in-character behavior. If the majority of the players have a problem with this, then you probably don't actually have a Collective campaign!
In a Competitive-Cooperative campaign, you need to remind the players that the new character should expect to be admitted, but probably not warmly; the player can expect his new character to get lots of in-character questioning or wisecracks and a "show me what you can do" attitude. Still, it should be a given that ultimately he'll be admitted to the party and play will go on.
In a true Individualist campaign, you don't need to do anything. Here the new character had better hope he's been created with knowledge, skills, or other utility to his prospective comrades, as the players are not beholden by any social contract to be nice to him. It might turn out that the party refuses to take the new character along. In response, he might follow them to aid them and prove his worth by backstabbing them during their next fight! Of course, if they fear backstabbing, the characters might kill their erstwhile comrade.
That's What My Character Would Do!
Imagine that a player's old character has died, and a new character has been introduced into the group, as discussed above. Unbeknownst to his new party members, however, this new character is of a radically different alignment than his new comrades, and during the party's next engagement, he ruthlessly betrays them, leading to the death of the entire party. "Why did you do that?" you ask. "That's what my character would do!" says the traitor. What do you do as GM in this circumstance?
If this happens in a Collective campaign, then there's been a violation of all of the implicit rules of the campaign. First, the player introduced a character who didn't fit into the collective. Second, by purposefully getting everyone killed, the player didn't respect the other players' right to enjoy the game. And third, by purposefully causing the campaign to end, the player breached the agreement to democratically decide major campaign decisions. You should stop the game, pull the offending player aside, explain that his behavior is out of bounds for this campaign, and rewind. A continuously problematic player should be ejected.