In short - set up the community group to always send out events on certain days at certain times. Events seem to always get people to pile into the servers, and I think that having a Steam pop-up to alert people of the server's, y'know, existence could do well to keep the server populations healthy. It's kinda something that I think (the lack of) caused the downfall of PF2's regular vanilla server - people only really filled it up when there was a scheduled event for the community group.