I wouldn’t necessarily agree that WhatsApp‘s spam protection is that great. I’ve been invited to quite a lot of pyramid scheme/scam WhatsApp groups, however that’s mostly happened after having to expose my private cell number on the internet (thanks to app stores and GDPR requiring some kind of phone number for businesses of any size).

An alternative few users chat app probably won't be a major target for spam untill it has lots of users.

So I would say it's a low priority feature in the backlog.

If your need is security then really that should be based on in person trust.

Or at least via a proxy.

So contact invitation can just be handled with use-once codes (or at least trivially burnable ones).