Why do people Have to join groups on fb just to get information about it? Trying to boost numbers to make it seem like it has more support? Trying to control posts on the group page?
Lack of experience with non-FB options?
Belief that everybody uses FB, thus it's the best place to reach the most people?
There's no single communications service that can reach everybody in a given geographical region, unless you happen to be working at the Civil Defense bunker.

Until there's a better option, FB has the numbers and offers the simplest apps to get in tough with other FB members. Sharing an event with 10 people can get you 1000 shares or more in a couple of hours.
It would be reasonable to add a newsletter signup on this site for members to opt in for event announcements, legislative news, and other discrete topics of interest. That way people who don't visit very often can get certain notices in a timely manner.
Something like that could easily be added to the signup process or profile settings.
Online tools are only as powerful as the owners are willing and able to make them.
https://wiki.simplemachines.org/smf/SMF2.0:News_and_newsletters