If you need to remove a member from a subject:
Click the Manage members button at the top right
Click on the cog icon at the right of the selected member
Select Remove from this subject
If needed you can search for a specific member.
You can also:
remove a subject's guest from the Guests tab
cancel an (unaccepted) invitation from the Pending tab
resend an (unaccepted) invitation from the Pending tab