Can I track the IP address of people requesting to join lists in GMS Communication Server?
Yes. GMS Communication Server adds an "X-Originating-IP" header to the list confirmation message. This way if you allow joins from any email address and someone is joined without their permission, the IP of the requestor is clear in the confirmation message.
Members are not joined to a list until they have postively responded to the confirmation message and this has been received by GMS Communication Server.
The "X-Originating-IP" header allows the senders of malicious join requests to be traced in the event that someone complains of receiving confirmation requests that they should not.