A group can call someone absent when they were never told the plan had changed.

Calling someone absent assumes they were told. Plans change in a message thread they were not on, a decision gets made among the people who happened to be together, and the person who never received the update is recorded as the one who did not come. The failure of communication becomes a fact about their reliability.

Before concluding someone opted out, check what they actually received. This is worth doing every time, because the correction is cheap and the assumption is expensive. And when plans change, take the extra thirty seconds to tell the people who were not in the room, since they are the ones who will otherwise be quietly reclassified.