whatever audience I need to send to and just update it manually as folks move in and out. It isn't ideal, but it works. I just put them in the bcc field when I send an email out. The most I ever need to send to is the entire elder's quorum which doesn't have that much turnover so it isn't that hard to maintain. If they are less-active I don't usually have an email address for them anyway, and if they are active, I'm usually pretty aware of when they move in or out.