Sending an email to all users:
Go to the Dashboard -> Users -> Select all Users -> Actions -- Send Notification Email. (This will send an email to all users in your organization that have not yet claimed their accounts - users that have already logged in will be skipped.)
Sending an email to certain groups at a time:
Dashboard -> Sub-Orgs -> Go to the sub-org in question -> Users -> Select all Users -> Actions -- Send Notification Email.