It is possible to add multiple contacts to an Event or Event Day from within the Event consult screen, or the Day consult screen.  

If you are on an Event and you go to the tab for either Staff, Attendees, Sponsors or Prospecting and click the Select Contact button, a window will open which will allow you to search.  

This window also has check boxes down the left hand side of the screen which you can select and, when used in conjunction with the Add to Event button at the bottom of the grid, will add the Contacts with the box next to them ticked to the Event.

On Event Days, the Attendees tab will have the same behaviour, though the button is Add to Day.  

This will also add the selected attendee to the parent Event, if they are not already linked.  The results for the Day attendee screen are defaulted to those contacts who are already attendees of the Event, so they can be added easily to a new Day which is created.