Account → Edit Profile holds your name, company, tax ID, address and phone. These details appear on your invoices, so fill them in before your first payment.
Keep the email address current above all. Invoices, ticket replies and renewal reminders go there, and an outdated address is the most common reason domains expire unnoticed.
Contacts add more people to the account, for example an accountant who should receive invoices. Users are people with their own sign-in to the client area and the permissions you give them. Use them instead of sharing your password - a shared password shares everything, including the right to cancel services.