The MCU has only introduced two of the guardians Illuminati, a cabal consisting of Reed Richards, Black Bolt, Namor, Charles Xavier, Dr. Strange, and, of course, Iron Man. The Illuminati was prominently featured in Brian Michael Bendis’s run on The Avengers.
The Illuminati was a clandestine group of A-list heroes that gathered in secret after major events. Following the Infinity Gauntlet crisis, it's decided that the Infinity Stones (which were referred to as Infinity Gems in Marvel Comics at the time) should be separated. Each member of the group takes one stone to hide. In Volume 4 of The Avengers illustrated by John Romita Jr., Parker Robbins learns of Black Bolt’s stone from an Inhuman he is in prison with. Robbins, also known as the supervillain The Hood, tracks down the stone and then goes on a quest to reunite it with the other five.
It is from this quest that readers learn where each of the guardians stashed their particular stone. Although the Inhumans have spent a considerable amount of time off planet, Black Bolt’s wife Medusa reveals that he would’ve never taken it off Earth. His stone, the Reality Stone, is hidden in the abandoned Inhuman city Attilan, located in the Himalayas. From there, Robbins goes to the Baxter Building, where Reed Richards has the Power Stone safely hidden in a pocket dimension of his own making. Using the Reality Stone, Robbins is able to manifest the Power Stone in his hand and continue his search for the others.
Namor, of course, hides his stone deep underwater while Xavier protects his inside a deadly simulation inside the X-Mansion’s Danger Room. Tony Stark buys Area 51 from the government after it goes on sale and chooses that as the hiding spot for his stone. Lastly, Dr. Strange hides his stone on the astral plane, where he wraps it inside the Crimson Bands of Cyttorak. Robbins is eventually able to locate all the stones as they call to each other as more of them are assembled.
So far only Iron Man and Dr. Strange have been properly introduced in the MCU, the latter choosing to protect his stone around his neck as the Eye of Agamotto. Although not a member of the Illuminati in the comics, Vision would be considered the Mind Stone’s guardian in the MCU. Technically, Black Bolt was introduced in The Inhumans TV series, though it’s unknown if that show is considered canon or if Stark would even feel comfortable giving this version of Black Bolt a stone. If and when the other of the Illuminati appear in the MCU, they will not have to protect any of the Infinity Stones, but the comics make it clear that Iron Man could not have chosen better guardians to keep these dangerous cosmic items safe.
