Allow "Keep in sync" without inititial member addition

[adamfranco]

When using very large groups for access control it is only necessary to track that the group members should have a role in the site, up-font addition of users isn't needed as the users will be added 1-by-1 when they next log into wordpress. An option could be added to the group-addition form to "Sync members on login" which would add the group to the synced list but not do the initial population step.