To remove people from a conversation see the How to remove people from a conversation article.
To add people to a conversation within Nucleus, navigate to a conversation where you would like to add someone.
When you start typing, Nucleus will start to offer suggestions based on the characters you've entered. An example of this can be seen below
Once you have selected the name(s) that you'd like to add to the conversation, click the Save button.
The person added will now see all previous AND future messages that are sent within the conversation.