I played a couple more placement matches, currently at three wins, one loss. Played McCree in two matches, Zen in another.
That "Stay in Group" button actually worked for once. After we sort of facerolled an Anubis game, we randos grouped up six-man style for a second game, where we in turn got bested by team of mostly gold and plat players. Ah well.
One of the guys in the group finished his ten games. He got around 2300-2400, so I imagine that's around where I'll end up
Nah, where you'll end up is based on your previous season's final ranking, and if it's your first competitive season, it will use your quickplay matchmaking rating which really could dump you anywhere depending on how much you practice things you're bad at in quickplay. You should end up near your previous season's rating or QP MMR no matter how you do on those matches, maybe drop a few hundreds if you do REALLY bad.
I really don't suggest using that
"stay as group" button in competitive, it's a trap. You're taking a team that was good at managing an uncoordinated or half-coordinated match against an equally uncoordinated team and making it into a large premade group that will end up fighting other large groups, a lot of which are coordinated. I tried it a couple times. I've had teams that just worked together so great and crushed the enemy just to be embarrassingly stomped once we turned it into a large group.
Don't do it if you value having a balanced match. Solo/duo queue and 6-stack are two completely different playing styles.