If you need to add attendance to dates that don't exist in Check-Ins, or you want to check in extra people after a session has ended, you can add people at one time. If the session doesn't exist, you'll need to create a session first, and then you can add people to the session.
If you are moving from a different database, or you need to add a session to the event, you can add past dates then add check-ins to that session. From the Settings, click Add past sessions.

Choose the date of the session you're adding as well as the time.

If you want to add multiple sessions that repeat weekly, check the box to add all the weeks up to the current session.
Select the event time as well as the timeframe of when people could check in.
Caution
Times do not default to your event time; make sure they're exactly correct before you Submit.
If the event date has passed, but you want to add check-ins to it, you can do it from the event session.
Check out this video to see how that works.
Make sure you're on the correct date, and then from the Check-Ins tab on the event, click Add check-ins to check in multiple people to the event.

Tip
Make sure you're on the right date by using the arrows or choosing a date from the calendar.
Choose the location, time, and type of attendee, and then type their name in the box.

Notice
Any warning messages you might have seen in Station mode will show underneath each person.
If you type a name of a person not in your database, you can check them in as a visitor. If you want them to be added to your database, you'll need to add them before checking them in.

Once you've added all the people to this list, select Check In, and the person will be added to the check-ins for that event.