Hi @Brady Oh
At present it is not supported to mention everyone in group chat. As a workaround, it recommends you add these users in Teams channel or mention each person in Teams group chat.
Meanwhile, we also recommend you vote for this feature in Teams UserVoice. Microsoft will always focus on customer’s feedback and experience. Some new features would be added to the services based on customers' feedback in the future, and your good ideas will be very helpful for them to improve the service.
Your time, understanding and cooperation will be highly appreciated.
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