Thank you for your question.
In the “Assign To” field in a ticket, you can specify any user from Office365 or AD, it will create appropriate contact item automatically.
The roles (Agent, Member, End-User) don’t define the rights of the users from Contacts list, they are just labels. Users rights are defined in the site settings, so you may restrict access to a HelpDesk site to specific groups of users. Thus, if you change a password or delete a user the changes will be applied for entire o365 portal including the HelpDesk site.
You may use Microsoft Flow to synchronize information from the Office 365 User Profiles with the Contact list. Please, take a look at this article to learn more about it.