I'd love to ask the elders this: when did you first start doing group chats, as in multi-user chat rooms with roughly permanent de-anonymized identities?
So, I was in jabber IM rooms in my first job, we didn't use cell phones, but there was an IT department chat, a companywide chat, etc.
Later technologies include lync/skype4biz, hipchat, teams, slack, and zoom.
Limiting myself more to the article's definition (mobile-first group chats), I'd say it was Google Hangouts, GroupMe, WhatsApp, then Signal, now its Signal + discord + FB messenger + all the millions of apps that just should be parts of libpurple / beeper.
But for those of you who remember eternal september, when did the group chat as those dot com babies like me know it first start out?