You will need to be a portal administrator to change layouts
Go to Portal Tools - Manage Portal Framework - Layout Manager
Click Assign layouts
Click the dropdown and pick the navigation layout you want to user
Move the users you want to be assigned to the layout to the right.
You can also move a role to the right, however, keep in mind, individual assignments will override the role. So if you have an individual assigned to a different layout, and they are part of a role, unless you remove the individual assignment the role will not work for them.