The following two APIs can not be called by apps:

• https://developer.atlassian.com/cloud/confluence/rest/api-group-group/#api-api-group-user-post
• https://developer.atlassian.com/cloud/confluence/rest/api-group-group/#api-api-group-user-delete

As per the conversation in https://community.developer.atlassian.com/t/adding-and-removing-users-to-from-groups-via-rest-api-via-connect-app/42654, the availability of these APIs would likely unblock the development of cloud app capabilities.

A similar request exists for Jira: https://ecosystem.atlassian.net/browse/ACJIRA-2260