... said Fabrice Roszczka, who works for one of the organisers, the Paul Guinot association that since the first world war has helped France's blind and visually handicapped. Here, 'who' is used to provide information on Roszczka, and 'that' to inform readers of the association's function. Below are some rules to bear in mind when using similar words:
who/that: subjects for people (Shakespeare was a prolific writer who is famous for his plays and his poetry.)
which/that: subjects and objects for things and animals (The plays that Shakespeare has written include Macbeth, Hamlet and Romeo and Juliet.)
who/whom/that: objects for people (I met an old friend last night whom I had not seen for a long time.)
when: for time, dates (1997 is the year when Hong Kong was returned to China.)
where: for places (This is the restaurant where he and I had dinner last night.)
Note that 'which' refers to things only, 'who' to persons only and 'that' to things and persons (but 'who' is preferred).