In D&D 5th edition's Forgotten Realms setting followers of gods are sentenced by Kelemvor and his servants to go to their god's realm after death (unless possibly they made a pact with a devil or similar.) This seems to be also true for primordials, celestial patrons, archfiends and some other powerful creatures. But presumably if a follower of a simple human charlatan dies he will not go to be with the charlatan after death. What is actually required for a creature for their religious followers to go to their realm in the after life in Forgotten Realms?
Owning an realm to be an afterlife is the necessary condition
There's a misunderstanding in the question. This question is akin to asking “what predicates an NPC being able to be king of a country in the Forgotten Realms?”
What predicates it is that you, the DM, have decided that they are a king and written everything else around that act of DMing creation, to justify and support your choice.
So too, what predicates a being controlling a planar realm that serves as an afterlife for its followers is that you, the DM, have declared it so. If you want a being to have a realm or a king to have a kingdom, you write that — and why — into your DM's notes.
A being that doesn't have its own planar realm, meanwhile, has nowhere for Kelemvor to send its followers and therefore he won't.
If a being isn't a typical god, and doesn't have the typical features of a god that would make it obviously earn a planar realm and the recognition of godhood by Kelemvor and yet you give it those things anyway, then that's fine. All you've done then is created a mystery: why is the planar realm of this primordial or whatever recognised by Kelemvor, and how did it create/gain the realm?
If you grant a charlatan a planar realm and the recognition of Kelemvor, then you've created a powerful mystery. (And made them not actually a charlatan — what a twist!) Write yourself the answer to the mystery, and then build that into your campaign.
If you (reasonably) determine that a charlatan isn't really a lord of a planar realm, then a follower of that charlatan is one of the Faithless when they are judged by Kelemvor. Sucks to be them. (And that's an excellent reason for charlatans to live in fear of discovery, and maybe consider not being a charlatan: their trickery not only scams their victims, but threatens their eternal souls too.)
There's no lore about this for things that aren't gods. Gods is easy: they can have a realm because, tautologically, gods have realms. (And what makes a god a god is that they have sufficient worshippers to make them a god.) For everything else — for any non-god with a realm that you, as DM, create — you're on your creative own for explaining that exception.
Q: What predicates an NPC having a house instead of not? A: You put it there.
Is your charlatan a religious leader following a god himself? If he is, then he and his followers all go to the same afterlife, because they are all worshiping the same god. Seeing as they are all going to the same afterlife then, yes, the probably show up around where he is, as he was the shepard of their flock, and if your god reserved you and your followers a nice little demiplane then you would also go there. Otherwise? If both you and your followers were faithless, then also yes. If you were different religions, though, then those religions take precedence.