Good morning,
Our group had been experiencing the same thing. We figured out pretty quickly on our end that each of our members (or a designee from each department) would need to contact their city's IT area and make sure that groups.io was listed as a "safe site" for the firewalls to allow messages through.
My group is for law enforcement agencies, though - and each jurisdiction has their own internet usage policy. I'm not sure if your group is similar.
Nicole Lambrecht