If you want to set up a forward to multiple people (also known as a "distribution group" or "mail group"), just separate the destination email addresses with a comma, for instance:

user2@domain.com,user3@domain.com,user4@a-different-domain.com

If you have a mailbox set up as well as the forwarder, the message will be delivered locally and forwarded (also known as "store and forward").

This only applies to users with clustered or Windows hosting. cPanel customers can achieve the same functionality by adding multiple separate forwarders.