Can anything be done with OLD groups?

Basically there was a group established in early 2009 that I am very interested in buying, it has mid xx,xxx members. It has been set to "private" for about 2 months although an admin can make it public again.

What can an admin of a group like this do? Is there any way to send out a mass update/inbox/anything?

Nope, 5k is limit for sending mails