I might join you later
What it really means
Usually a soft "no". "Might", "maybe" and "possibly" in answers to invitations often mean the person will not come but does not want to refuse directly. If they truly plan to come, they usually ask for details like time and place.
Context, tips & common mistakes
Treat "I might join you later" as perhaps a one-in-three prospect of attendance — it is the standard exit from an invitation the speaker does not want to refuse outright. The social contract has two halves: they get to decline gently, and you agree not to wait for them, save them a seat, or mention it afterwards. Nobody is offended when 'later' never arrives; raising it ("you said you'd come!") is the only move that breaks the etiquette.
It is also yours to use: it declines warmly while leaving the door open, and if your evening changes you genuinely can turn up. With close friends, though, plain speaking beats code — "I'm shattered, not tonight" lands as honesty, and honesty is allowed once a friendship is real.