This seems to be the hard part of your requirements:
As I understand it, and have been using it ...
a. You CAN give another webid access to a domain that's hosted in your account. See the panel, Billing > Account Privileges. [That would allow your friends to, say, set up and change e-mail addresses on that domain.]
b. You CANNOT give a second ftp user access to the files in the first ftp user's directory. To really share the files, they need to share the user name/login. See
But, as described in the article you cite on mapping subdirectories, you could distributed the site to the different users by giving each person one or more subdirectories that they could control with their ftp id.
Perhaps if you reword your requirements, someone will have a better answer.