Issue Summary

It's not possible to add users to a local group through the Organizations API in a siteless organization.
This is especially concerning for Trello Enterprise customers who have migrated to the new Admin Hub experience, as they can no longer manage Trello product access via API.

This is reproducible on Data Center: n/a

Steps to Reproduce

1. Create an organization with only siteless product(s) (for example, Trello).
2. Sign up for Atlassian Access.
3. Create a local group.
4. Try to add a user to this group using the Organizations API (https://developer.atlassian.com/cloud/admin/organization/rest/api-group-directory/#api-v1-orgs-orgid-directory-groups-groupid-memberships-post) without manually invite these users before

Notes: there's no API to invite users to siteless product(s)

Expected Results

Users are added to the local group through the API

Actual Results

API response - "USER_NOT_FOUND". Instead of using the API to handle hundreds of users, the Org Admin needs to invite the users manually.

Workaround

Sign up for at least one sited product (for example, Jira Software)
Invite the users manually (https://support.atlassian.com/trello/docs/inviting-people-to-a-workspace/)