As administrator, simply login and go to Administration/Mailing lists.

Then create a mailing list (ex. you can call it 'all') and in the addresses section, select an item from 'Add group addresses'. You can then add all users in the system, all users in a domain, and so on.

Then you can set the permissions for the mailing list, to decide who can send to this mailing list (maybe only you, or some users, or everyone, etc).

Then, to send an email to all users in the list, simply send the email to all@mydomain.com , or whatever the name you chose for the list.