Noah came with the message and warned his people saying:

(  إِنِّي لَكُمْ نَذِيرٌ مُّبِينٌ… ) (…indeed I am to you a clear warner): He warns his people against the torment of the Hereafter, and calls them to worship Allah (swt). Indeed, the warner is the one who warns of bad matter which time has not yet come, so that the listener might be prepared to face it. As the Prophet Noah (PBUH), he has come as a warner, so the context continues; because the previous verse says:

  مَثَلُ الْفَرِيقَيْنِ كَالْأَعْمَىٰ وَالْأَصَمِّ وَالْبَصِيرِ وَالسَّمِيعِ 

(The example of the two parties is like the blind and deaf, and the seeing and hearing…) (Hud :24)

Thus, there is a disobeying party to whom Allah (swt) sent Prophet as a warner. On the other hand, the Prophet is a bearer of glad tidings for the believers.