Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Pages 86-87
Question: 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?
Response: If they did not catch [any of] the prayer for rain [in congregation], then they have missed [their opportunity]. However, if they arrived whilst the imaam is giving the sermon, then they are to offer two rak’ahs upon entering the masjid before sitting down to listen to the sermon and uttering aameen during the imaam’s supplication. [Importantly], they are not to offer the prayer [for rain individually or] in congregation as they have missed [their opportunity].