Did you enable groups for the bot?
use @BotFather, send it command: /setjoingroups
check the bot's privacy mode,
did you add the bot as an admin of the group?
See also,
https://stackoverflow.com/questions/37338101/how-to-add-a-bot-to-a-telegram-group
and,
https://stackoverflow.com/questions/37243303/telegram-bot-doesnt-respond-in-group-chats
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