According to the Hanafi madh'hab, praying Fard behind a leader is only valid if the Imam is also praying Fard.  It is, however, permissible to pray Sunnah or Nafil behind a leader praying Fard.

You can never know whether the person is praying a Fard or Sunnat or Nafil salat, as it is by niyat (intention). Hence touching an already praying person and starting one's own Fard salat is risky unless you know for sure that he is also praying Fard.