[ndriscoll]

You don't need a centralized approver; you could give any member invite privileges or have a shared password or QR code that members can give out, or that can be shared at in-person meetups, etc. That should work fine still for a city-scale group (which in practice is still going to be maybe a few dozen members). Community forum like for a PTO is also the exact type of thing I had in mind. Print a QR code with embedded password on flyers.

[graemep]

Its still going to create extra costs and risks. What happens if a member over-shares a QR code? What happens if you trust the wrong people? Children can scan flyers too. Governments will move to close loopholes and get ISPs to block sites. Technical solutions do not fix social and political problems.

[ndriscoll]

This is certainly an American perspective, but I wouldn't worry at all about such things. The government has no legitimate interest in regulating your local bicycle club's discussion forum so you should be about as worried as you'd be not going to church if a law were passed saying everyone must be Christian.