Mandates allow you to have granular control over what is shared within a project. They give you greater control that simply states or individual sign types do.
To access the mandate controls for a project, right click on the project in the Left Hand Sidebar and select "edit".
Under the "Mandate" tab you will be presented with options to control which locations or sign types will be shared with users who are given access to the project.
The first thing to do is select the "Use mandates" checkmark. You can then select from the dropdowns which locations and sign types users will be given access to. 
Once you've saved these settings, it will apply to all users who do not have admin access to the organisation.