To set security levels to issues user must have access to Set Issue Security at project's Permission Scheme.

In Issue Security Scheme create Issue Security Level to allow access to secured issues only to granted groups, users or roles. 

On "Add User/Group/Project Role to Issue Security level" screen you can find CRM's Security groups:

Security GroupUsers who will have access to issuesHow to set
Company employeesReporter's colleagues (from same company in CRM).How to set Reporter's colleagues access to issues 
Holding employees

Reporter's colleagues, employees of parent company, employees of all child companies and

employees of companies which have the same parent as reporter's company

Add Parent and Child companies attributes to CRM Company
Parent companies employeesReporter's colleagues, employees of parent company and employees of parent company of the parent companyAdd Parent and Child companies attributes to CRM Company