They are at a public meeting, in a government building, exercising their 1st amendment rights to petition their government. They have absolutely no privacy rights here. They are not compelled to identify themselves but they have no protection against others identifying them nor are they likely to receive such protection given the legitimate concerns the public has over corruption (the Lobbying Disclosure Act [2 USC 1605] does permit lobbyists to perform routine information discovery, such as attending this event, without requiring disclosure but the fact that they are not required to report their attendance would probably not be enough for a court to find any expectation of privacy for anyone attending such an event.)