If you want to send a mass email to a specific group of affiliates you will need to navigate to Emails> Send Message in your Merchant panel.
Then click on Define another filter as showed in the screenshot below.
Now you are in the Affiliate Manager and you will need to click on the advanced filter.
Now click on "Add custom filter field", to choose the parameter, which will be used for selecting the
Here you can choose it.
Then please save the filter and you should to see it in Emails>Send Message.