Simple example: with Slack (or another hosted, web-based chat app), I can invite the non-technical head of sales to join it via e-mail, and they can. They literally cannot install an IRC client.
I can get similar results, but "I" isn't enough for a communication tool to be useful.
Why can't you do it through the web interface of a company-backed IRC network? There's literally no difference there, IRC is just a protocol, you can still wrap around your own local stack with registration, email notification, fancy UI (I know UI != UX) and auto-invite system or whatnot.