Author names should be separated by a semi-colon, like so: Brian W. Kernighan; Dennis M. Ritchie. Don't include the word "and" or anything similar, even if you have 20 authors. If you have the auto-format option checked, then the last name will automatically be shown first for each author.
Other properties which allow multiple values, like "genre" and "keywords", are entered in the same way, with a semi-colon (;) separating each value.