The OED’s first example of the collective term lobby meaning “persons who frequent the lobby of the house of legislature for the purpose of influencing its members in their official action” is dated 1808.

The original lobby was the one attached to the chamber of the British House of Commons, in which members could meet and talk to outsiders. This sense (and function) is recorded from the middle of the seventeenth century and was adopted in Congress when it was established more than a century later.