Mailman is probably the easiest way to get a mail list (group mail) setup without using a wiki to do so. It's in Server Admin --> Mail --> Settings --> Mailing Lists. You can open up the users and groups list and just drag people into the list. It'll setup a "Mailman" box first, which needs to be there, and then you can setup whatever mailing lists you need. Emails to these addresses are formatted as <list_name>@your_server_domain.com. It's actually more of a full-fledged subscribe/unsubscribe type list, but it'll do the job.
Kind of the long way around, but you could log into their webmail account and setup the rule there.
I found the solution by looking at very old server configs and how I used to do it back in 10.1/10.2 days.
user > su -
su > vi /etc/postfix/aliases
# Put your local aliases here.
groupname: user1, user2
su > postalias /etc/aliases
su > postfix reload
An that is it.
I do have a corresponding WGM entry for groupname for local access controls, but if you just need a mail group alias, this will work by itself.
This will also work for my 2nd question above about redirecting mail from user to another (assuming account is deleted in WGM and not just disabled).
This process is less than ideal though and a huge step backwards in functionality and administration of mail
If anyone has a WGM or Server Admin solution, I would love to hear it
if the account is deleted from WGM, it should be fully removed from dovecot as well. ie: a mail to email@example.com would get a rejection reciept.
To redirect mail for the deleted user, you can do the following follow the above steps with the following info
#Put your local aliases here.
deleted_user_alias: redirect_alias1, redirect_alias2, firstname.lastname@example.org
So you can redirect to 1 person or multiple or even external domains. just ensure all aliases and addresses are seperated via a comma.
Note: This will only work if the alias has been completed deleted (and the user no longer exists is dovecot!)