[Mailman-Users] postfix + mailman / Gentoo = user unknown in local recipient table

[pvmailman at nekophile.com]

Postfix works.  I've added the aliases entry.
I can create lists fine.  The aliases go into the local alias file for MM:
/usr/local/mailman/data
root at twiggy $ ls -la
total 68
drwxrwsr-x   2 mailman mailman  4096 Jul 16 15:15 .
drwxrwsr-x  20 mailman mailman  4096 Jul 14 22:28 ..
-rw-r-----   1 mailman mailman    41 Jul 16 12:56 adm.pw
-rw-rw----   1 mailman mailman  9185 Jul 16 14:58 aliases
-rw-rw-r--   1 mailman mailman 24576 Jul 16 14:33 aliases.db
-rw-r--r--   1 mailman mailman    10 Jul 14 22:28 last_mailman_version
-rw-r--r--   1 mailman mailman 14110 Jul 14 23:07 sitelist.cfg

However, when I attempt to mail one of the lists, I get:

[Mail not sent: <list.cal at nekophile.com>: User unknown in local recipient 
table

If it helps, when I enter the relevant list data into the 
/etc/mail/aliases file, I get


<mylist at nekophile.com>: Command died with status 2:
    "/usr/local/mailman/mail/mailman post list.cal". Command output: Group
    mismatch error.  Mailman expected the mail wrapper script to be 
executed as
    group "mailman", but the system's mail server executed the mail script 
as
    group "nobody".  Try tweaking the mail server to run the script as 
group
    "mailman", or re-run configure,  providing the command line option
    `--with-mail-gid=nobody'.

I've done the manual config fine before, but this is a Gentoo ebuild and 
I'd like to be able to keep up to date easier with 'emerge.'  

Any thoughts?