WhatsApp carries the invite; it does not carry the follow-up
A group chat is a genuinely good place for a webinar: people read it, they trust the sender, and a link costs nothing to post. What is worth understanding before you send is the division of labour. WhatsApp is the delivery channel for the invitation and a signal to you, the host, that someone registered. Everything that happens after a person signs up runs on email.
So the flow is: you post the page link, a colleague registers on the page, you get a WhatsApp message telling you a name just came in, and that colleague receives a confirmation email with their private access link, then email reminders before the session. Nobody gets nudged about the webinar in the group chat, and no guest ever needs a WhatsApp account to take part.