Ridiculously easy and cheap to start out. Helps you organize events, meetings, and mostly get people started doing paddling together.
Be aware that a meetup is like a pickup ball game: Whoever shows, goes, but often people don't take it quite as seriously as a commitment as you might like. So you may get many no-shows, or folks who come only once or intermittently.
I started an informal biking group 26 years ago with a buddy. It grew and morphed. Some stayed for decades, some didn't fit in from the get-go. Others came and went. In the end we have a tight group of 11 friends, who may differ at times, but always pull together. It's a bonding group that is very special. However, I also started the group's annual outings, and believe me, organizing those was a real bear! And I herd cats for a living...
So the less formal and the less you expect, the more likely you are to succeed, IME.