1- Add the bot to the group.
Go to the group, click on group name, click on Add members, in the searchbox search for your bot like this: @my_bot, select your bot and click add.
2- Send a dummy message to the bot.
You can use this example: /my_id @my_bot
(I tried a few messages, not all the messages work. The example above works fine. Maybe the message should start with /)
3- Go to following url: https://api.telegram.org/botXXX:YYYY/getUpdates
replace XXX:YYYY with your bot token
4- Look for “chat”:{“id”:-zzzzzzzzzz,
-zzzzzzzzzz is your chat id (with the negative sign).
References
https://stackoverflow.com/questions/32423837/telegram-bot-how-to-get-a-group-chat-id