Those are problems that can be left alone until they actually become issues, which would probably be the case once the hypothetical service has become successful; in that case, there should be enough cash flow and minimal overhead needed to implement/hire moderation. But if you're spending all your money on building rent, fancy office equipment, lots of bureaucracy, etc., then of course moderation is going to become difficult.

These problems can be mitigated early on by charging meetup organizers. If a $20 a month is too much, then it probably doesn't belong on a meetup site. It's enough that anyone willing to abuse the system will take the path of least resistance and use Eventbrite to host webinars.

Any other meetup communication should be made unappealing enough that moderation rarely becomes an issue. It should be possible for people on meetup to communicate, but it should be a second class citizen to however the organizers communicate with group members. Any forums that are part of meetups should suck just enough that nobody wants to use them unless they are serious about communicating something.

There are creative ways to handle moderation that don't require obscene amounts of money or employees.