Yes, Krewe does drop you into a group as soon as you sign up and you don't get to see who's in it. Keep in mind that we never give ANYONE your location or email address. You also have the choice to leave the group right away and join a new group and you have the ability to vote people out of a group.
But I strongly believe the platform simply won't work if people are able to pick and choose who they want to be in a group with. It's not hard to imagine that men will only want to be in a group with attractive women. If people can choose, then they'll only choose "the best."
I believe that the issues you raised apply to essentially anything in life. Every time you go to the grocery story, you run the risk of bumping into someone you don't like. If you go on Tinder, you can come across your ex. If you join a recreational kickball league, you could be put on a team with your arch nemesis. Krewe doesn't make you interact with anyone, it just give you the opportunity to do so. You should always meet up with new people in a safe, public space.
With that said, I will be making some small changes that I hope will help address your concerns.