GoToConnect allows you to add a third number to an ongoing call. See How can I add a third number to an ongoing call?
If you need to have more than three phone numbers participating in a call, there are two options:
Departments can get a "conference bridge" which is a phone number that up to 99 people can dial into at the same time to participate in a conference call.
See https://support.goto.com/jive/help/how-do-i-join-a-conference-bridge-jive-join-conference-bridge for step-by-step information.
Go to https://services.dartmouth.edu/TDClient/1806/Portal/Requests/ServiceDet?ID=40274 to request a Conference Bridge for your department.