Not the person who wrote that reply but their reply immediately clicked for me.

Here in Europe basically everyone uses WhatsApp. So you might as well just place a WhatsApp status update writing something like "We're doing X this afternoon? Wanna join? We'll leave from [location] at [time.]" and basically everybody in your contacts can see that update.

It's not perfect (different privacy model, a lot more people can see the update unless you have a very curated contacts list) but no friction of getting people to install another app.

Edit: Also no notification for status update, so slightly different model that could solve the same need.