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 employees

Contacts from any company specified for the issue. If the user is a contact of any CRM company specified the issue in the relevant custom fields, he/she will be granted with access to the issue.

If you use only the CRM Company field and assume the Reporter is the contact from this company, this security level will grant access to the Reporter's colleagues.

How to set Reporter's colleagues access to issues 
Company assigneeAssignee's colleagues (from the same company in CRM). If selected, this security level checks if the Company attribute of a user has the same value as the Assignee's one.
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 the parent company and employees of the parent company of the parent companyAdd Parent and Child companies' attributes to CRM Company