Sometimes your members might bring in guests and you want to check these guests in, too. Be it in order to charge the member additional day passes or just to correctly track attendance.
While the check-in button disappears after a member has checked in you can check her in multiple times on the same day time in the Check-in calendar. To get there go to the member's page and click on "Past Check-Ins" in the Check-Ins panel.
You can then check in the member by clicking on a day.