The situation
A reply arrives to a gathering invitation.
English meaning: Unfortunately, I can't attend this time. Please give everyone my regards.
Read the Japanese original
残念ですが、今回は出席できません。皆さんによろしく。The questionWhat does the closing call for?
Read the explanation
Clues in the scene
Unfortunately, I can't attend this time. Please give everyone my regards.
Attendance is addressed explicitly before a separate request to pass on greetings.
Each choice explained
Accept the absence and pass on regards — Best fit
The answer declines attendance and asks for greetings to be conveyed. Those are two separate instructions to preserve.
Count them as attending because of the greeting — Does not fit
Regards for the group are not acceptance. Counting the speaker as attending removes the earlier refusal.
Automatically reschedule the event — Does not fit
No date change is requested or confirmed. Non-attendance alone does not establish a rescheduled event.
Treat the entire reply as sarcasm — Does not fit
No preceding conflict or tone supports treating the whole message as sarcasm. The explicit refusal and greeting can be read first.
Same words, another scene
After sending this refusal the day before, the person sends a new message saying their commitment was cancelled and they can now attend. The organiser rereads the earlier wording in the record.
The earlier message still declines attendance, but the later message changes the attendance record. Keep what the first reply meant separate from what the person has since confirmed.
I will check the numbers using your new message that you can attend.
Literal meaning
The speaker cannot attend this time and asks that greetings be passed to the others.
What these words cannot tell us
The reason, strength of regret and desire for a date change are not known from this answer.
A question to ask in real life
I will mark you as absent and pass on your regards.