Answers
What is the best app for a neighbourhood group?
Something that does not require exchanging phone numbers with people you have never met. A Naven channel or a Discord server both work; a WhatsApp group hands every member's number to every other member.
Neighbourhood groups are the clearest case for this, because the membership is by definition people you do not know well. A WhatsApp group of forty neighbours is forty phone numbers distributed to thirty-nine strangers, permanently, with no way to withdraw one.
The second requirement is that joining is easy for people who are not technical. A link that opens in a browser clears that bar; an app store download and an account often does not.
A Naven channel is a link. People open it, sign in with an email address, and are in.