Active subscribers are current (not expired) subscribers who will be sent alerts to any active/validated devices in their account.
View a list of all "active" subscribers on the People >> Subscribers >> Active menu.
Subscribers who opt-in will become active when they create their account.
To make one or more "active" subscribers into "inactive" subscribers, select their name(s) from the list and then select "Inactivate" from the menu at the top of the list.
(Inactive subscribers will not receive alerts.)