Extranet users can be invited by Bitrix24 employees. Bitrix24 employees can discuss questions in the Extranet workgroup or project with the Extranet users, work with files and documents, add tasks to Extranet users, etc.
The table shows a comparison of the capabilities of intranet and extranet users, grouped by the functionality they have access to:
You can invite Extranet users on any Bitrix24 plan, but they are counted as business tools users, the number of which depends on your Bitrix24 plan. Read more in the article - Business tools.
For the Bitrix24 On-Premise version, Extranet users are not counted as business tools users, and you can invite an unlimited number of Extranet users.