To add people to a conversation within Nucleus, navigate to a conversation where you would like to add someone and click on the Add person to conversation button indicated in the example below:
Next start searching for the person(s) you'd like to add to the conversation by typing in the search box. When you start typing, Nucleus will start to offer suggestions based on the characters you've entered.
Once you have selected the name(s) that you'd like to add to the conversation, click the Save button.
The persons added will now see all previous as well as future messages that are sent within the conversation.
Recommended: How to remove people from a conversation