Missed the prayer for rain

With regard to the prayer for rain, if a group [of people] enter [the masjid] and [find] the imaam is giving the sermon, after already having just completed the two rak’ah prayer [for rain in congregation], should they [now] offer the prayer [for rain themselves] or should they sit down [with the rest of the congregation] for the sermon? Also, is [the rain prayer] to be made up [if missed]? And [if it is], should they offer the prayer in congregation?