Can't add user to scope in databricks - User or group doesn't exist

Question

I'm trying to add a user to a scope using the databricks CLI

databricks secrets put-acl --scope <my scope name> --principal <user's email address> --permission MANAGE

Although the user has access to the workspace and has been using it (and there is another user added to the scope via their email address) I receive the error:

Error: b'{"error_code":"INVALID_PARAMETER_VALUE","message":"User or Group <user's email address> does not exist."}'