Do you just mean via the administration login for SmarterMail? If so there are a couple of ways you can do it: 

- Impersonate the user account you want to be shared out and share each of the resources you want to extend to other users. You can do this via Settings>Sharing or via the individual sections like Webmail/Calendar/etc. You can extend sharing to groups or individual users with varied levels of permissions. 

- Add a domain-level public folder share and replication the permissions noted above. This will be available at the domain level and doesn't require maintaining a separate account. 

If you mean via a control panel like WHMCS or Plesk that process could vary significantly. 

I cant share the inbox afaik.