yeah, federation is a quite wide topic. in our case (rocksolid), we just feed all the messages received on the websites into newsgroups, and all the new messages from the newsgroups into the websites (so we bridge nntp and http). the postmill author(s) considered federation at one point (although with a different method), but dropped this part of the code later on.

btw: my original account was not accessible for me any more, so I created trwII.

cheers

trw