Cobb, Ariadne, Fischer, Saito all die in the dream and end up in the same Limbo world, which is the world Cobb and Mal built.
Does this have any logical explanation according to the rules set up by the film?
This is explained in the movie in the scene where limbo and the potential death of a character during the mission is first mentioned (after Saito got shot on level 1). When Ariadne asks what limbo is, Arthur explains it to her:
Unconstructed dream space. [...] Just raw infinite subconscious. Nothing is down there. Except for whatever might have been left behind by anyone sharing the dream who has been trapped there before. Which in our case, is just you. [to Cobb]
To understand how this is possible, we don't really have to look too far beyond the already accepted concept of shared dreaming. In fact, it's an established concept that people can subconciously bring their own stuff from their very own mind into the shared dreamspace. The best example is Cobb and his subconcious projections of Mal and their kids. But afterall, also Ariadne as the architect practically brings her own world with her (albeit more conciously than Cobb) even if she isn't actually the dreamer.
The point here is that the limbo space that everyone shares is pupulated by what Cobb left there in his own subconcious. But it's not that there is an actual limbo filled with all that stuff that people once created there and that's magically stored away in some aetheric dream realm independent of anyone's mind. Rather than that, the mutual subconcious limbo that is itself shared by all the people sharing the dream is just populated anew by anything that the people currently in the dream once created and stored in the subconcious part of their very own mind.
So the limbo is basically the connected subconcious minds of all the people in the dream (in the same way their dreams are connected), just that Cobb is the only one who has already filled his own subconcious with anything meaningful. And in fact when Cobb finds Saito at the end (beginning?) of the film, we see that Saito has been there long enough to build his very own little realm.