The xMatters integration for ServiceNow provides the ability to quickly assemble an xMatters-hosted conference bridge using the Engage with xMatters feature. However, there is no ability to add additional groups/useres to an existing xMatters-hosted conference bridge using that feature. This was due to limitations of the xMatters REST API. The API has now been updated to support the underlying requirements and the update set provided here allows the Engage with xMatters feature to take advantage in order to supporting adding groups/users to an existing xMatters-hosted conference bridge.
Note: This feature is now available in the 4.0.2 ServiceNow integration! So if you've upgraded to that version, no need to install this!