Is it possible for external contractors who do not have profiles on our company's HR system and thus are not included in the regular feed to gain access to the portal for training purposes?

You can create a dedicated domain for external users who are contractors, and their accounts can be managed manually. A designated person would be responsible for tasks such as archiving, updating, and reactivating these accounts as needed. Despite being separate from the main staff, these users can still be included in reporting or can be reported on as their own entity.
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Speak to your assigned customer success manager about your requirements in futher detail.