Masbooq refers to the person who joined the Imaam after the Imaam had completed one or more rakaats. Thus he will  complete the missed rakaats after the Imaam makes salaam.

When the Imaam completes his Salaah, the masbooq should pause slightly after the Imaam has completed BOTH salaams. Thereafter he should stand to complete the missed rakaats (Shaami). 

If the masbooq forgetfully made salaam with the Imaam He should stand up for the missed rakaats immediately upon remembering, provided that he has not committed any act which terminates the Salaah. If  he had made the salaam simultaneously with the Imaam, he should then merely complete his salaah in the normal manner. If he made salaam after the Imaam had uttered the word “As-Salaam” (which is generally the case), he should make sajda sahw at the end of his salaah (Shaami).
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