If the contacts are in the Event already as Invited use the Contact List under the Event Modules to open the Event Contact Details form. If the contact(s) are not already attached to the Event you can open the Event and access the Event Bookings to make the required bookings.
Event Contact, Membership and Order modules now have two menu options: Recalculate Discount and Remove Applied Discount (under Book menu in Event Contact, under Discount menu in Membership and Order)
Each module also has a Discount ID and Discount Desc field which are populated if a discount has been applied for that record
Membership has a discount comment field so this will update the field whenever the discount applied changes