I think the last time this was done was by Torqez in 2009 or so.  Few organizations and businesses don't email their customers at some point asking them to join something, get involved, visit, etc.  The opening day of round starts, why not send out an email to all former players (perhaps a filter can be done to limit the email to those that have completed at least 1-2 rounds to avoid re-inviting inactives), and invite them to join for the current round.  An opt-out option could also be included in the email, so that if they plan on never playing again, they can opt out of future round invitations.

Just a thought for filling and better balancing galaxies pre-round.