Conversations form naturally around actual groups of people, and change as the group changes. Concerts, night clubs, protests, parades. No need to create chats or send invitations. Just start messaging.
Chats are also visible to other people nearby. Find out what's going on, as it's going on. Upvote events to spread word of the event farther out, even around the world.
No usernames, no signups, no fuss. Have fun, be nice.