In order for this to work the external system would need to have a webhooks mechanism to make calls to JIRA to access the endpoint and retrieve the users, parse that data and format it into JSON to add to the payload.
You can create users, groups, etc. via the REST API and add the users to those groups. Here are the resources for the REST API in JIRA:
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG