Currently, when using the below endpoint in order to add users that have an accountId starting with "qm:<string>" to a group, the request fails with the return code HTTP 500, instead of one, which would indicate why the specified user(s) can't be added to the specified group. It is important to mention that such accounts are most likely "Portal-only customers" that don't consume a license seat, and hence can't be added to JSW groups.

https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-groups/#api-rest-api-3-group-user-post

Steps to replicate:

make an API call to https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-groups/#api-rest-api-3-group-user-post while passing "groupName" and "groupId" as parameters and the "accountId" (or multiple Id's) in the body.

Actual results (may vary based on language preferences):
{
"errorMessages": [
"Internal server error"
],
"errors": {}
}

Expected results:
Any sort of an error message that would allow the admin/person executing the call to understand why the request failed.

Workaround:

1. Follow this article in order to export users having an Atlassian account
2. Filter the account search, for instance, this one to list "accountType": "atlassian" only. This should be done after retrieving the list of users.