How do I add a new contact within a transaction?
To add a contact within your transaction, from its page select Action > Add user / contacts. To add an existing user simply type their name into a field. If you want to create a new user, start typing their name and then click the Create new link. In a pop-up window, fill out the necessary fields of your new contact, select the needed role and click Add.
How do I delete a contact from the transaction?
Select the contact within a transaction and click Revoke access on the right-side panel.
How do I change a transaction owner?
Select the contact within a transaction and select the Transaction owner checkbox button on the right-side panel.
How do I create or edit a contact?
Go to Contacts > Add contact. and proceed with filling out the needed info. If you want to edit a contact, click the Edit button on the right-side panel.
You should have a Can manage private contacts permission to manage contacts.
Public vs Private contacts

The below video will be useful for the users with the Admin level of access.