Get Group members in ADMIN API (not Confluence, not JIRA)

I want to get the group members just like when you access the admin portal in https://admin.atlassian.com/.
So not over any Jira or Confluence API since the group was also created in the Admin portal.
I only see ways to add members, remove member and list groups.

Is there really not API to get the group members or what? This should be basic.