Users can be managed in the "administration" tab of the vCloud and in the "Users" section in the "Members" category:
Once in this menu you will be able to add users and define rights they have on your vCloud organisation. The rights can be given depending on 5 categories:
Organisation administrator: he has all rights on the vCloud and he can for example create users, define their roles, create vApps, VMs, etc...
Catalogue author: he has all rights to create vApps, catalogues, import templates or virtual machines. He can manage vCloud with no Administration.
vApp author: he can create one or more vApp(s) with VMs inside and configure the vApp network.
Console access: this user can only access to the console of the virtual machines to administer them and it's the administrator, who defines the vApp and the virtual machines that are accessible.
Please be aware that for each user you can limit the number of resources that he can consume on the vCloud.