If this is for a New User, you'll be prompted for their Email address and Name, and optionally their timezone. By default the system will send them an email asking them to set their own password.
If this is for an Existing Account Contact then their information is already in our system and they will continue to use their existing sign-in information to access the control panel and their quarantine area.
A single quarantine user login can be configured to see an unlimited number of email addresses. This functionality can be useful if a single person has multiple email addresses or if several users want to share a single quarantine. A group quarantine setup can be useful for small teams for departments that want to be able to share the ability to review the quarantine.
The billing cycle for your domain can be changed by going to the Account > Subscriptions tab. Find your domain and then click on the associated CHANGE link in the Next billing column.
See also: Can I add quarantine users in bulk?