I have rode many times as a couple, no strangers...I wonder if they do put you with others if you only have, say, two people?
If it is busy they will load parties of 1, 2, or 3 with other parties of 1, 2 or 3. There is no way to guarantee your own car but you have a better chance during a low crowd time. They usually announce this while you are in line over the PA.
If you have 4, you will get your own car. Since they seat 3 per side, they will not make you sit with strangers on your side.
I could never ride this with anyone besides my family. I don't need strangers seeing me in such a blubbering state of fear.![]()
but my youngest loves it so I braved it for him. I asked the cm if we could have our own gondola she told me no. I told her I would really like to have our own and she rolled her eyes at me but did let us be by ourselves. (That honestly was the one of the only times a cm was rude to me but I just let it go. Most are wonderful so not going to let one ruin it for me.
) It was hard enough for me to go on that darn ride, I didn't need to be sitting next to complete strangers. It was not busy at the time, the line was only about 10 to 15 minutes. 
Getting matched up with other folks isn't always bad. I know a couple that got seated with another party...turns out that the other party were Club 33 members and they were asked if they had been on RSR yet and ended up being giving Fast Passes RSR good for later that night!
No word if they made enough of a connection for a reservation!