I'll be glad to clear things up! Indeed, on a shared server, we cannot allow multiple FTP/shell users to have access to a single domain directory. It is possible (though still not recommended) to set it up on a VPS or dedicated server, following the instructions here:
The best and most simple solution is to create a new user in your panel at Users > Manage Users, for each of your hosted domains that you require multiple admins to have access to. For example, if domain.com has three admins that work on it regularly, just make sure it is the only domain hosted under that username. This way, you can safely share the login/pass credentials for that user to the three admins, and they will only have access to that domain.
If some of the domains you need this set up for are already hosted under a user alongside other domains of yours, no worries! You can have our panel move a domain to another user for you automatically. Just head over to your Manage Domains page and click "Edit" to the right of a hosted domain. In the Fully Hosted settings there, you can choose one of the new users to host the site on (or create a new one there). A little box will appear, asking if you want all the content for that site moved to the new user. Keep the box checked and save/change your new Fully Hosted settings; it will move everything under that domain to the new user for you.
As always, please feel free to contact our tech support DreamTeam if you have any questions or need any help setting this up: http://dhurl.org/dhsupport