All of them will appear in On Call Now - that is, ANYONE in any on call calendar right now.
However, the person that receives the alerts is determined by the escalation policy for that group. You could have a calendar that's not in the escalation policy but has people in it, might be handy for just having a backup schedule or if you were doing some manual processing. If you want someone (on call or otherwise) to receive an alert, you either have to manually assign it or they have to be in the escalation policy.
To see the escalation policy for the group, go to Groups -> Edit and scroll down a bit past the members.
Thanks, that all makes more sense now