You can set permissions at various points in your customer business hierarchy.
To set permissions for a specific customer's Case or Ticket process, go to the Service Matrix, select the Permissions link and add one or more User Groups in.
Note: any Permissions set at higher level will show read-only when accessing settings for any item in the Service Matrix.
Setting User groups with permissions at the other levels in the business hierarchy can easily be done by navigating to the Edit screen at the relevant level.
Once that has been set, only Agents within this User Group has access to work items (creation, editing etc.) within that part of the hierarchy. For example, if you link up at the customer level, that User group has access to all work items under that customer. Conversely if you set down at the case process level, that User group has access to work items within that case process specifically within that customer/contract/service combination.