This is a simple automation that will provide alert (sample given via Telegram) upon player joining or leaving.
[Prerequisite] Minecraft Server & Minecraft Server Integration (https://www.home-assistant.io/integrations/minecraft_server)
[Sensor used] sensor.minecraft_server_players_online, sensor.minecraft_server_players_max and sensor.minecraft_server_map
[Installation] Add new automation, edit in the YAML.