If you don't know the name of a guest you can just check the Anon check box. A name will then be created for them automatically, based on their group name.
Or you can import anonymous guests using a Num. guests or Num. in group column in the 
Import Data window:
Or use "+n" in the 
Quick Add Guests window:
You can uncheck the Anon box and add their name if you find it out later.