https://app.element.io/ -> click "create account" -> create account. The default is for a matrix.org account which is totally fine for anyone who can't/won't dig deeper.
This is the level of technical ability that you need to be targeting.
Re "you said it was called Matrix": fair, but I wouldn't send people to "matrix" / the protocol for any system with multiple implementations. I'd send them a link to one of the user-friendly apps (i.e. Element). Similarly, you wouldn't tell someone to join XMPP, you'd send them to the Google Hangouts or MSN Messenger or Playstation apps.
This is a friction-filled experience for someone coming from WhatsApp
Also, search still doesn't work in encrypted rooms.