If they absolutely have to meet in person and cannot telecommute, and cannot be connected to each other via finances/ shared interests etc. then I would suggest they each meet in a modest suite in a hotel near an airport every month.
Assuming these incredibly powerful people got that way by pre-planning everything, then they will have their next monthly meetings planned out for at least a year in the future. This means that they can have their people booking these suites under different aliases in order to not be connected to each other.
There can also be a myriad of reasons why these people would all be present in the same location at the same time at one of these sites: vacations, book tours, business meetings, layovers whilst waiting for a connecting flight, passing through whilst driving from one place to another. Then they all can go to this same location for different reasons (eat at the restaurant/ drink at the bar/ booking a room).
This means that they would not be connected via similar functions/ interests, as there wouldn't be records of them all in the same place at the same time for the same thing, so they wouldn't be connected that way. It would simply be mere coincidence that all of these people are nearby at the same time, and wouldn't be connected to each other unless someone already knew all of their identities.
There are also an enormous amount of locations around the world in which they could meet. They could meet in a different country every single month and still wouldn't have gone to the same country twice after a decade.
And there would be dozens of hotels available in each of the airports in each of those countries. Booking a larger suite (not the largest because that might make people curious as to who is staying there) means that there would be enough room for them all to interact comfortably with each other and wouldn't arouse suspicion.
In addition, if the hotel is at least reasonably large, having various strangers wandering about would not arouse suspicion, as there will be a heavy number of strangers there for a short period of time (particularly if it is near an airport and is used to having people stay for only a single night).
This also allows for the opportunity to have some of them be famous people. If they are the Illuminati, they can't all be pulling the strings, some of them would have to be play-makers, hiding in plain sight. So even with some of them having a moderate level of fame (published books, CEOs of mid-level companies etc.) it likely wouldn't raise suspicion if their paths crossed from time to time.