Witnesses who provide testimony at Congressional Hearing can be a good way to identify interest groups on a policy.
In addition to being the official transcripts of House and Senate floor activity, the Congressional Record contains pages numbered "HL" which is know as the Lobby List. The list is published quarterly in February, May, August, and November.
A lobby list identifies individuals and organizations that lobby Congress and the clientele that those lobbyists represent. The list is based on disclosure forms filed with the Clerk of the House and Secretary of the Senate, and it is published in the Congressional Record quarterly, usually in February, May, August, and November. The three sections of the lobby list are registrations, quarterly reports for the most recent quarter, and quarterly reports from the prior quarter that were received too late for inclusion in the previous list. From GPO Access Congressional Record Search Tips.