It strongly depends on who is in charge, if you are in control of the community you can excert force and move people. Shut down an existing group, move it to read only at Facebook and so on. If you are not in charge, all bets are off, it is very very hard to convince people that sugar is bad for them.
Perhaps @carlfranklin could let us know a bit more about how they made the switch, https://www.ketogenicforums.com was a spectacularly successful Facebook groups move.