By “congregation of Muslims” he actually means the experts of independent reasoning (ahl al-ijtihad) and legal answers in the obscure matters which require insight and investigation, as well as the agreement of the Community of Muslims concerning what is obligatorily known of the religion with its decisive proofs.
Shafi`i continues (Risala p. 253): “The Prophet’s order that men should follow the Muslim community is a proof that the Ijma` of the Muslims is binding.” Later on (p. 286) he quotes the hadith whereby the Prophet said: “Believe my Companions, then those who succeed them, and after that those who succeed the Successors. But after them falsehood will prevail when people will swear to the truth without having been asked to swear, and testify without having been asked to testify. Only those who seek the pleasures of Paradise will keep to the Congregation…” Shafi`i comments: “He who holds what the Muslim Congregation (jama`a) holds shall be regarded as following the Congregation, and he who holds differently shall be regarded as opposing the Congregation he was ordered to follow. So the error comes from separation; but in the Congregation as a whole there is no error concerning the meaning of the Qur’an, the Sunna, and analogy (qiyas).”