One of conditions of proper Salat Al-Jama'ah ( Congregational Prayer) that Imam Al-Jama'ah must be a pious believer in The Twelve Infallible Imams (AS). Nevertheless, it is allowed to pray with Muslims from any sect, but you need to recite Al-Hamd and Sura for your own with out clear voice. Ismaili sect is a Muslim sect.
Wassalam.