Users have full account access unlike Suppliers and Buyers where access is restricted to a specific portal. There are two type of Users - Administrator and Standard. You can restrict Administrator rights however the standard user access is restricted by default.
Admin > Users.
Only Administrators can add users. You can add a new user and determine the access rights as well as restricting access via IP address (such as your office or home network) or country.
For details on how to do this:
You can create a group and assign users to a group. This is predominantly used if you want a group of users to have access only to certain campaigns.
Campaign Permissions (users and user groups)
By default, campaigns are visible for "everyone" (all users). You can restrict campaign access to certain users or user groups. For example, you may wish to only give access to 1 campaign to a specific group, every other campaign would be invisible.