I don't get group chats. It seems like the worst parts of IRC had a baby with the worst part of internet forums and created a thing destined to make you always late to the party.
And woe betide those who get into a group chat with a bunch of chatty people who never stop texting. Then you turn off notifications for it and you miss all kinds of things.
what did it take from internet forums that IRC couldn't have realistically already done?
Most group chats to me are pretty much the same as IRC was back in the day, unless you examined the protocols themselves. Upload limits are also much larger (except for that brief period skype was popular and you could upload gigabytes)
Honestly the only thing group chats are bad at is archiving, and it's mainly because it's designed for real-time conversation (so people are spamming more) and sites can't search the stored text at all.