If the congregation forgot to blow Shofar [at all, neither before nor during Musaf] and only remembered after completing the Chazan’s repetition, they are nevertheless to blow the Shofar upon remembering and are considered to have fulfilled their obligation.
 593/4; Rama 593/2
 If however they blew before Musaf and forgot to blow during Musaf then there is no need to blow after Musaf as they already forfeited the ability of blowing in accordance to the order of the blessings. [See previous footnote] Practically however the custom is to always blow a total of 100 blows even when Davening in private. [Piskeiy Teshuvos 592/3]