Can I add/remove users to my Business Plan in the middle of the billing cycle?
Of course, you can add more licenses at anytime during a billing period, and you can assign them to users as well. Your account will be charges a prorated amount based on the amount of time left in your billing cycle when a new license was added.
Alternatively, you can remove a user from existing license and add another user to it if necessary, but the license itself can only be removed at the end of the billing period.