If you are on the business plan, you are able to have multiple users in a single conversation with a contact.
To invite another user to the conversation, you will need to access the contact's conversation. On the very bottom, you should see a "+" icon in which you are able to invite another user to the conversation. You can add multiple user to one single conversation.
Please note that once an user is invited to the conversation, they will remain in the conversation until the user has voluntarily leave the conversation.