There may have been an announcement by Rachel, but Finn knew that when Rachel called a club meeting, no one showed up. Well he did. He liked seeing her happy, but he seemed to be the only one.
Finn was prepared for the worst. Either he'd have to console Rachel when no one showed up and he'd be dragged around trying to find everyone in an attempt to convince them to come to the meeting so Rachel wouldn't wring him by the neck oooor they were all locked up in cages and Rachel swallowed the key.
He stared back at Mr. Shuester, hoping he'd realize just how bad this situation could turn out without the right amount of damage control.