After hearing about the miraculous things Hashem did for Bnei Yisrael when taking them out of Mitzrayim, Yisro (Moshe's father-in-law) travels from Midian and comes for a visit.
I would say it's almost universally accepted that Yisro ended up converting and joining Bnei Yisrael when he comes and talks to Moshe about what happened.
If he did convert, then why does Moshe send him back home to Midian (18:27); shouldn't he have stuck around with Bnei Yisrael, especially since they were to receieve the Torah soon at Har Sinai?
(Of course, the Torah is not in chronological order so it would be easy to say that Yisro was sent back home after they received the Torah, and that the Torah only puts Yisro's visit and return here as a matter of reader convenience.)
Why was Yisro sent back home instead of living in the desert with Bnei Yisrael and eventually entering the land, and more importantly why did Moshe do this? Because the passuk says "vayshallach Moshe es chosno" - Moshe sent his father-in-law away.
The Mechilta (as referenced by Rashi) writes that he went home to convet the other members of his family, but does this imply that he returned, along with his new converts, back to Sinai?