In early drafts, Kyle Reese was sent back with a second person, who he finds having transported inside a fire escape and died. He calls his companion "Sumner" when discussing the future with Dr Silberman (due to the strange nature of the timelines in the Terminator franchise, it's unclear if this is the same Sumner who actually appeared in The Sarah Connor Chronicles)
Reese: The Terminator had already gone through. Connor sent two of us, then zeroed the whole place. Sumner didn't make it.
In the released film, it's worth noting that John doesn't "send" Reese back (At least, Reese doesn't think of it that way). Reese specifically comes back to meet Sarah Connor:
Sarah: So, was it John that sent you here?
Reese: I volunteered.
Reese: It was a chance to meet the legend, Sarah Connor...
Reese: John Connor gave me a picture of you once. I didn't know why at the time. It was very old. Torn. Faded. You were young, like you are now. You seemed just... a little sad.
I used to always wonder what you were thinking at that moment.
I memorized every line, every curve.
I came across time for you, Sarah. I love you. I always have.
Note that in the second quote, John Connor gives Reese a picture of his mum as a young woman. (A little creepy if you ask me) The implication of this is that Connor already knows that Reese is his father, and is specifically sending him back because he knows that it works.
When we see the time travel scene on screen in Terminator Genisys, it plays out the same way, and Connor is clearly preparing Reese to meet a woman who might not live up to his expectations. Also note that that film shows what might happen if they don't destroy the time machine quickly enough: the machines in the future can send back more terminators to an earlier point.