The Bilderberg Group is a real organization, and has met annually since 1954. Most information about it, from attendees to topics of discussion, has been confidential, which has led to a great many conspiratorial theories about the group and its goals.
The issue states that the conference is only for certain countries, and that the king of Wakanda, an African nation, was extended a special invitation to attend. This is essentially true. The group was originally intended as a meeting of Western European and North American representatives, but reports indicate that it has extended slightly beyond that. Russia and Turkey, as well as other Eastern European nations, have become regular attendees, and China and Kuwait have supposedly had representatives in recent years. (Though there's no suggestion that Japan has ever been invited.) So despite the secrecy and a limited guest list, it's quite plausible for Wakanda to be invited in.
For more on Bilderberg, there is a good article from BBC News about the group and its air of mystery. Or you can google around for the conspiracy sites, but I don't think they merit a link here.
And while I'm on the subject of BP, let me say a word about Everett K. Ross. In the new series, 'Ross' appears to be an agent of some US intelligence agency, reporting to the NSA. But when Priest was writing him, Ross worked for (and commonly referred to) the US State Department's Office of Protocol, which is also a real agency. According to its webpage:
The Office of Protocol...directly advises, assists, and supports the President of the United States, the Vice President, and the Secretary of State on official matters of national and international protocol, and in the planning, hosting, and officiating of related ceremonial events and activities for visiting heads of state. The Office also is the administrator of Blair House, the President's official guesthouse.
Which ain't far off from how Ross described his job.