To add a person to a private chat in Teams, follow these steps:

1. Check the membership of the conversation: Make sure the person you are looking to add is a member of the Teams chat conversation.


2. Check if the person has been blocked: If the person you are trying to add has been blocked by any member in the conversation, you won’t be able to add them.


3. Check your app version: If you and the person you are trying to add are using different versions of Teams, you may not be able to add them to the document.


4. Check if they are an external user: If the person you are trying to add is an external user or from another organization, they might not be able to join the conversation.


5. Check the group settings: The Office 365 admin or the Teams admin might have disabled private chat self-add feature.