The new feature in data bricks is identity federation if identity federation is enabled then the users part of the data bricks account and the account admin can assign the users to the workspace. The account admins can add the users from account console. Please follow the doc on how to assign users to workspace from account console.
"https://docs.databricks.com/administration-guide/users-groups/users.html#assign-a-user-to-a-workspace-using-the-account-console"
if the account is not enabled for the identity federation then a workspace admin can add the users directly in the workspace admin console.
https://docs.databricks.com/administration-guide/users-groups/users.html#add-a-user-to-a-workspace-u...
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Once check the status of the users in the workspace admin console it's active or not ?
You can also check using SCIM API.
https://docs.databricks.com/dev-tools/api/latest/scim/scim-users.html#scim-api-20-users-for-workspac...