I think most of this should flow from your overarching community strategy.
Why are you bringing people together?
If the purpose is to foster group cohesion, then small 10-20 people whatsapp groups could be a valid strategy. They are incredibly powerful in creating a feeling of community. A small in-group like this can generate cohesion that something in a larger setting cannot.
But there are a lot of downsides too!
If your purpose is to generate content, increase your SEO, distribute information, or if you have minors in your community.. probably not as great of an idea.
What’s your overarching goal? 