It is very simple:

1) Move the Listserve user list over - 400 users is nothing - get the master list 
and do invites. If 20 did 20 invites - done. Those that can type faster, have at it.

2) Everyone who wants to be an Admin can, forums all over the internet function 
by self-policing.

3) Those who have Google Groups (reg or enterprise for edu) enabled in their 
domain will have amazing functionality for sharing with the Group.

4) Profit - Congratulations, you have evolved to the next level. Use the time 
saved to slowly get a cup of coffee or soup.