manage_membership for unix groups provider

I searched and didn't see this feature request so if is/was at some point forgive me.

I'd like to be able to define the users that belong to a group in the group's resource. My compelling reason is that a specific unix group is used to determine ssh access via AllowGroups in sshd_config, and I'd like to make sure that at all times only specific users are in that group. My workaround is to manage /etc/group as a file resource, which isn't very puppet-like.