Is group dua after prayer a bidah?

What the Prophet ﷺ did after the prayer

After the salam he would ask Allah's forgiveness three times and say "Allahumma anta as-salam wa minka as-salam, tabarakta ya dhal-jalali wal-ikram" (Sahih Muslim 591). Ibn Abbas said that raising the voice with dhikr when people finished the obligatory prayer was known in the Prophet's ﷺ time (Sahih al-Bukhari 841). He also told Mu'adh never to leave, at the end of every prayer, the dua "O Allah, help me to remember You, thank You and worship You well" (Sunan Abi Dawud 1522). So dhikr and dua after prayer are firmly sunnah. See What should I say after every prayer?

The question is the fixed group form

In many mosques, the imam turns around after every fard prayer, raises his hands, makes dua aloud, and everyone says "Ameen" before wiping their faces. The debate is about making that exact form a permanent part of every prayer.

• Those who discourage it (scholars such as ash-Shatibi, a Maliki, and Ibn Taymiyyah, a Hanbali, and many who follow them) say the Prophet ﷺ didn't do it regularly, so treating it as part of the prayer's routine adds something to worship.
• Those who allow it (many scholars in Hanafi and Shafi'i communities, especially in South Asia, Indonesia and West Africa) point to the general encouragement of dua after prayer and of raising the hands, and say a group saying "Ameen" together is a good thing as long as nobody thinks it's obligatory.

Most agree that an occasional group dua, for example after a lecture or for a specific need, is fine.

What should you do as a new Muslim?

Follow the practice of your mosque calmly, or quietly do your own dhikr if you prefer. Don't argue with the imam or call people innovators in the prayer hall; manners matter more here than winning a point. If you want to make your own dua, raising your hands is fine, and you can do it in your own words. See Do I have to raise my hands when making dua? and Should I wipe my face after making dua?