Patrick King has set up two Chicago Region groups using the Telegram messaging app.
One group, Rides CRBMW, is for instant notification by anyone planning an impromptu ride. The other, Chat CRBMW, is for general communication amongst members.
Telegram (https://telegram.org) is available for mobile devices using iOS or Android, and also for Windows, Linux, and MacOS desktops.
Here are web links to get you to the groups:
https://t.me/+YeRCdAurFuszNjVh gets you to Rides CRBMW
https://t.me/+Tu2tKmxvWfY5NGEx gets you to Chat CRBMW
Your cell number is your login (but is not revealed to any other user), and so far the service is free.