I suspect the OP is not a speaker of English and is having difficulty with expressing himself. When he said he joined the line, he may mean that he tried to check in. Other passengers had already checked in, but the agents had already left and no one was there to give him his boarding pass. They had not yet boarded the plane so he might have expected to be able to get his boarding pass.
He doesn't state the time when he got to the check in counter. He got to the airport an hour before the flight. He also doesn't say what airport, but I find it extremely odd that an AA counter an hour before a flight leaves would be completely unmanned especially the Saturday after Thanksgiving. The check in time for domestic travel is 30 minutes before departure for most airports but Dallas would be 40 minutes.
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