Did they? It seemed to me like she just decided, "I'm gonna go find Luke, seeing as I already got his lightsaber and I've got questions," and nobody seemed to argue with her. Finn certainly didn't know she was important to the Resistance, seeing as he was unconscious.
She had R2, and she had a tracker given to her Leia. This would seem to suggest that she was picked of this job. However to the bigger point.
Rose-Thats how were going to win. Not fighting what we hate. Saving what we love
This is literally what Finn was trying to do when he was going to leave, he was saving Rey. Next when he was running to the gun, he was trying to save everyone else. If Luke didn't show up they would have been setting ducks .
Finn wanted to save Rey and get away from the war. It was a selfish reason. Rose saved Finn because he's become one of the best Resistance soldiers, and him sacrificing himself on a gambit that wasn't likely to succeed (like everything else in this movie) would've been a waste of a good soldier and a close friend. She knew that the Resistance needed as many allies as they could get, not to fight today, but for the chance to fight another day.