Unlimited plans are billed based on the number of Copilot user logins in your Copilot account. For example, if Geoff and Katie can each login with their own email address to your account at any point during the month, you will be billed 2X the per-user pricing. You can find a complete list of the current pricing plans here. Only one session per login may be active at a time. In other words, Geoff and Katie can both have a session active at the same time, but if Geoff shared his login with Dave, he wouldn't be able to start one as long as Geoff had his active.