I kind of think this would be an issue no matter what platform was chosen. I have people who prefer to talk to me on Google Hangouts, but I don't check that as often as Facebook Messenger, and similarly, my messages to them via Messenger go unnoticed for longer periods of time. Then, since I'm also a Waze map editor, I'm on Discord, because they switched from Google Hangouts about a year ago.
The only sure way to work this would be group SMS texting, but that has its own issues.
And announcing ANYTHING on-the-fly would be problematic due to distracted driving. On the last (and only) roadmeet I attended, I was alone in my car with three dogs, having originally planned to attend with a passenger. I could have taken on passengers, as there was considerable car-switching during that meet IIRC, but I did not know anyone and did not realize that was an option. Plus, I was afraid my dogs would be less than friendly (which was an irrational fear; they just bark a lot with new people for a few minutes and then calm down).