After you have created a new Deal, navigate to the Participants section. A popup will appear with the participants (along with their role) that are currently part of the deal.
Click Add to start adding participants.
Whether you add an existing contact or invite a new someone not in your database, they will receive an e-mail alerting them they have been added to a deal.
From your end, select the role of the participant from the dropdown menu and now everyone can start working on the deal!
Please note, at this time five participants can be part of one deal.