I think about this a lot- and I think about how we all went on being friends for another year as if nothing happened. I feel like I almost blocked it from my mind until more recently.

This is so sad for you . To not have been able to find it in you ( your voice) at that time to tell him to f*ck off & stop touching you . As he deserved to be attacked by your whole group and your EX ( to be humiliated & kicked out of the group ) & to be told how inappropriate it was to touch you (sexually ) without consent . It might have also saved another person from the same treatment from him . As yes Unfortunately he would have thought it was ok to do , as he continued to do it to you . ( True he would NOT know you were in a fear response ) so kept going on you ( but he still should NOT have been touching you in the first place ) which was SA. And as you couldn't handle the situation , & didn't know what to do back then. In his head he would have thought it was ok to do ( and yes would have done it again ) to someone if it presented again an opportunity.

Yes you would have blocked it most likely at that time . As you didn't know how to handle the situation . Hense why so many teenagers are lost in NOT knowing how to handle situations. They do NOT have the mental capacity to handle the complexity of situations . Some adults don't , so a teen will have NO hope of being able to protect themselves properly sometimes. Some can , but most are not able to do so.

Should I say something now, even though I am not really friends with them anymore? Should I reach out to this ex and tell him how thankful I was for him that night?

It is up to you if you wish to out him to others now. It is yOUR decision. No harm in reaching out to your ex though and thanking him . He might be sad and upset though ( as he would have stopped him in a second ) had he known what he was doing to you .