Free beer. (Stolen from "One Froggy Evening".)
I suggest one (1) polite reminder by email at most, and posting on appropriate metas and newsletters at StackExchange. I recommend divvying up the list of people to be contacted among those avid enough to participate here and have each of those mail the people on their portion, to show existence of a community worth joining. While some people who signed up may not care any longer, hopefully most have just forgotten or have not gotten the news. Also, ask them what it would take to get them to join. Some of them might know.
Gerhard "Just Checking Out This Place" Paseman, 2015.09.11