go to the web panel and in the left hand navigation click through Users > Users
on that page click the link at the bottom of the main area [ Add New User ]
from the next page that loads you will be able to create the new user, pass, and designate them as a shell or FTP only type user
Once you've set up a new user (this may take a while before this user can actually login via FTP/shell) you can go again to the panel's left navigation and click through to Domains > Manage. Once that page loads you should see your list of domains, under the one you'd like to make available to the new user you'll need to find the row that says "Web (Site is 'mirror'.)" (or "Web[Upload To Site]" depending on whether the domain is currently fully hosted or mirroring another) and follow that row to the right where you'll click the [ Edit ] link.
That will take you to the page where you will want to designate the domain as "Fully Hosted" and set user dropdowns for the "web Directory", "Logs Directory" and " All CGI should run as user" spots to the newly created user's space.
Again, this action will take some time to get up to speed and fully functional.
You will now be able to give that user the keys to this isolated house.
If you find that the new developer needs more control over the account and need to allow him/her access to your web panel (to manage databases or somesuch), that can be done as well, but that's another long post