Append a line in BASH only on certain lines

I am working on a script to make changes to a Nagios plugin service definition using BASH. I need to append the contact group name line but only for certain service definitions. So I would start with this.

define service {
    use                     sites-service
    host_name               my_host
    service_description     check_reboot_os_updates
    check_command           check_reboot_os_updates
    contact_groups          contactgroup1
    servicegroups           MyGroup
    }
    
define service {
        use                     linux-service
        host_name               my_host
        service_description     other_description
        check_command           other_command
        contact_groups          contactgroup1
        servicegroups           MyGroup
        }

And I want to append only select contact group lines. So say I wanted to add an additional contact group to the Linux services like this.

define service {
        use                     sites-service
        host_name               my_host
        service_description     check_reboot_os_updates
        check_command           check_reboot_os_updates
        contact_groups          contactgroup1
        servicegroups           MyGroup
        }
        
define service {
        use                     linux-service
        host_name               my_host
        service_description     other_description
        check_command           other_command
        contact_groups          contactgroup1, contactgroup2
        servicegroups           MyGroup
        }

Is there a way I can do this using sed or awk or something else?

Using sed, if the string linux-service is unique, you can try matching from the line containing the string to the line containing the string contact_groups appending the additional group within the match.

$ sed '/linux-service/,/contact_groups/s/contact_groups.*/&, contactgroup2/' input_file
define service {
        use                     sites-service
        host_name               my_host
        service_description     check_reboot_os_updates
        check_command           check_reboot_os_updates
        contact_groups          contactgroup1
        servicegroups           MyGroup
        }

define service {
        use                     linux-service
        host_name               my_host
        service_description     other_description
        check_command           other_command
        contact_groups          contactgroup1, contactgroup2
        servicegroups           MyGroup