What is the ruling on the congregational prayer of the muezzins who prayed one rak'ah then joined the regular imam who was in the first rak'ah, knowing that the Maghrib prayer is three rak'ahs, and thus the muezzins' congregation would have prayed four rak'ahs?

The mentioned muezzin may move to follow the regular imam. The congregation behind him becomes invalid if they continue to follow him after that, even if ignorantly. Their prayer is valid if they intend to separate from him and follow the regular imam, according to the Shafi'is. As for the Malikis, they consider the prayer of the muezzin and his congregation invalid if they do so. If they intentionally add an extra rak'ah, their prayer is invalid according to everyone. If the addition was due to forgetfulness, the prayer is valid, and the regular imam bears the forgetfulness if it occurred after following him.

The Shafi'is hold that an imam may follow another imam, whether he intends to separate from his congregants or not. If an imam follows another imam, the following of the first congregants behind him becomes invalid. If they become aware immediately, they must separate from him; otherwise, their prayer becomes invalid if they continue to follow him. They may follow the one whom their imam followed, or complete their prayers individually.

As for the Malikis, they stipulate the intention to follow at the beginning of the prayer, not during it, and they hold that transitioning from praying individually to praying in congregation or vice versa during the prayer invalidates it.

Therefore, if a cause that invalidates the prayer of the muezzin and his congregation has occurred, they must repeat it. If only a cause that invalidates the prayer according to the Malikis alone has occurred (which is following the regular imam during the prayer), then, out of caution in religion, it is advisable to repeat it to avoid the disagreement among scholars.