Unlike some games STO has it set that the more people you have on a team the harder a mission will be.
So please add an option to kick a player because why should the rest of us have to work harder because a player joins the instance making it harder only to camp around afk!! I for one dont want to be working harder to benefit a lazy git!
if you're the team leader you can kick players, so they beat you to the punch its in there. If you're not you can leave the group and change instances. if neither of those solutions appeases you open up you social options window "O" and change your auto group settings to what ever you like.