My understanding is groups and communities are for people who already know each other directly or indirectly, like a mutual friend or parents of students of the same class.
WhatsApp for Business is something separate from this.
Yes? Phone numbers are not top secret information. My schools always had a student directory that listed every student's home phone number and their parents' mobiles. We used to have phone books that listed everyone in town's phone number.
Back then, phone calls were expensive and my time/attention was cheap. Now phone calls are cheap and there's so many things I'd rather do than answer robocalls and anxious parents.
WhatsApp for Business is something separate from this.