There is also a hidden file called ".qmail." that can be edited to forward email to as many addresses as you like without doing mail groups.
On my box, this file is located at:
/var/qmail/mailnames/domainname.com/"email address"
Use Plesk email forwarding to set up the first address, then edit "/var/qmail/mailnames/domainname.com/emailaddress/.qmail"
The file should look something like this:
| true
&firsrtemailaddress@somewhere.com
&secondemailaddress@somewhere.com