By editing Attendance Sessions and adding new attendees as visitors, it's possible to mark their attendance even after attendance for the group has already been posted.
Step 1: Edit the Attendance Session That Has Already Been Posted
Go to Attendance > Reports > Sessions and select the Attendance Session that the new attendee needs to be added to, then select the gear drop-down and choose "Edit".
Step 2: Add the New Attendee to the Session as a Visitor
Select the "Add Visitor" button and search for the existing record or create a new record if the person is not currently in the database.
The individual is added to the Attendance Session as a Visitor attendee and "Total Present" is increased. Note that the individual has not actually been added to the group.
Step 3: Edit the Newly Added Group Visitor and Update Attendance for the Session
Select the newly added Visitor record
Go to the Attendance Tab and click in the "Present?" column next to the Attendance Session date, change the value from "Visitor" to "Present" or otherwise as appropriate. The change is instantly saved.
Step 4: If the Attendee is a "Permanent" Group Member and Should Appear on the Group Roster, Add the Group to the Individual's Record
Go to "Edit" and under the Groups section add the group.
Be sure to "Save" to complete the update.
The Individual Now Appears Correctly in the Prior and New Sessions