In a space, type @all in a message and select All: Mention everyone in the space.
You'll get a message asking you to confirm sending the message as an @mention to every person in that space.
Webex Teams | Get Everyone's Attention in a Space
If you have something really important or time-sensitive to share with everyone in a Webex Teams space and want to make sure they read your message, type @all in the message. It works as if you @mention every single person in the space individually and notifies them of being @mentioned. But be careful not to overuse this feature, though!