Send group SMS to specific user role (Zapier/Twilio Integration)

I am trying to create a form that enables a user (User Role: manager) to send a broadcast SMS message to a group of users (User Role: employees). I am using Zapier and Twilio to accomplish this. 

The employees User Role has a phone number field. I need to enable the manager to select from a list of employees (by name) by clicking a checkbox beside each name, which would then pull the mobile phone number of each selected employee and populate ALL phone numbers into a field on the form. This field is then configured in the recipient field when setting up the Zap. It will have multiple phone numbers separated by commas. 

I have created a many-to-many connection between my "group message" data object and "employees" user object. My problem is that I can only seem to pre-populate with user names, but have no way to get the phone numbers into a field that Zapier will pull phone numbers from to use with twilio.

Any ideas?