To set security levels to issues user must have access to Set Issue Security at project's Permission Scheme.
On "Add User/Group/Project Role to Issue Security level" screen you can find CRM's Security groups:
Users who will have access to issues
How to set
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.
Assignee'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.
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
Parent companies employees
Reporter's colleagues, employees of the parent company and employees of the parent company of the parent company