When you are going through the Journey, you can easily assign contacts to to the roles that are in the template by simply clicking on the box underneath the role card name and selecting the appropriate contact. If you have not already used the contact, you can enter the information and select "Create new contact". If you have used the contact before but are adding some fields that were not previously used, you can save those fields back to the contact by clicking "Update Contact".
All of your saved contacts can be viewed by clicking on the Contact icon from the Platform.
The Template as a Play Analogy Continued
Remember our analogy of the template as a play? If we think of the different cards as the characters of the play, the contacts are the actors. There can be an actor for each character but an actor can also play multiple characters in the play. Contacts can also be assigned to multiple roles in your template or template set.