It is correctly pronounced "yu-SOO-poff" with the accent on the second syllable.
Generally in Russian words and names, but not always, it is the second syllable which is stressed (tol-STOY, ro-MAN-ov, shu-VAL-ov. Unless, in a multisyllabic word, the root syllable of the original word is present in other than the secondary position. (STRO-gan-ov, OR-lov, etc.)(Never mind. Too complicated for here.)
The name Iusupov (or Yusoupoff, as the family spelled it) is drawn from the name of their ancestor Yusuf, who converted to Orthodoxy (see Greg King's biography of Princess Zinaida Yusupov on the site). The "ov" ending in Russian is roughly the same as the french particle "de" meaning "of" Yusupov can be roughly translated as "of Yusuf" This is not true for every name, and is a very general rule.
"Sky" at the end of the name can mean the same thing. For example, the Princes Beloselsky were originally from Beloselsk. Also, illegitimate members of the Romanov family were known as "Romanovsky" meaning "of the Romanovs"
More than you asked for, sorry. And remember, there are many exceptions.
Best,
nick