I'd be really bothered by that, but my first act would be to ask them why they invited my ex-metamour to a small group event that I attended when they knew I had an acrimonious relationship with them. You might want to stay open minded, as you may hear some things you weren't expecting that make you think about if there are some changes YOU need to make. You might just find out your friend is inconsiderate or oblivious.
At the same time (if you think the people who host the event want to have only people who will get along swimmingly at it) I may send a note to the people in charge of inviting others or not and ask to talk to them about the nomination of said ex metamour. It seems odd to invite or be attempting to vet a new person into a group when you've only been a member for a few days and haven't met everybody else yet.
Happiness will never come to those who fail to appreciate what they already have.