Update 2025: Aaaand for me it does not work anymore in the browser app...
Update 2024-05: The official "New Teams" browser app now shows a popup notification when someone starts a meeting.
To show a popup notification when a person joins a meeting, I use Teams for Linux. It is an
Unofficial Microsoft Teams client for Linux using Electron. It uses the Web App and wraps it as a standalone application using Electron.
To install: use the script, snap or flatpack on their GitHub (I used deb-get for simplicity).
sudo wget -qO /etc/apt/keyrings/teams-for-linux.asc https://repo.teamsforlinux.de/teams-for-linux.asc
echo "deb [signed-by=/etc/apt/keyrings/teams-for-linux.asc arch=$(dpkg --print-architecture)] https://repo.teamsforlinux.de/debian/ stable main" | sudo tee /etc/apt/sources.list.d/teams-for-linux-packages.list
sudo apt update && sudo apt install teams-for-linux
Small note: The behavior is different than Windows. On Windows you also get a notification at the time the meeting is scheduled.