Mahajanas are the twelve delegated agents of the Lord Vishnu who have the duty to bring to the people the message to follow the path of devotional service. The word means "great persons", and was also used for administrators or officials of the Western Chalukyas rulers.
Bhagavata Purana (6.3.20-21) lists twelve Mahajanas, biggest devotees of God Vishnu and knowers of Bhagavata-dharma. They are Lord Brahma, Narada, Shiva, the four Kumaras, Kapila — the son of Devahuti, Svayambhuva Manu, Prahlada, Janaka, Bhishma, Bali, Śuka, and Yama.
References
Mahajanas Wikipedia(Text) CC BY-SA