“Regular chat” like the one that is installed with the phone when you buy it. Not sure what Apple calls it but on my google phone it is simply called “messaging.” Every phone has it installed and the people you are wanting to keep in touch with won’t have to download, install, and get an account to use whatsapp or signal.
I have been thinking of starting a similar group..using texting...but that seemst limit viewing unless things get passed on...but, maybe that is good...or I could put together a group addy?