Once you have logged in as an administrator (found beneath the "Help" menu when you are logged in as a user) select the company and then the project you want to add users to.
Under the "Project" menu select "Add users to project".
Click on "Add" for the users you wish to be a part of the project. Don't forget to add yourself!
Note that if you do not see all of the users who are part of the project it is because you have not been given administrative rights to all of the companies involved. If you need to be made an administrator for a company other than your own please contact CtrlPrint support.
You can now assign user permissions which will allow users to access the documents. More information about how to do this can be found here.