Everyone and their mother know the core four members of the Fantastic Four; Reed Richard's Mr Fantastic, Sue Storm's Invisible Woman, Johnny Storm's Human Torch and Ben Grimm's Thing. Over the years, every member of the team has been subbed out for various narrative reasons, but mostly to shake up the team roster.
There isn't much the team hasn't done. From space travel, to undersea adventures, even reaching into the realms of mysticism. So, it is unsurprising that the FF has reflected these aspects with various members of the larger Marvel universe lending their support across the team's long history. As Dan Slott highlighted in the third issue of his current Fantastic Four run, Marvel's first family has an expansive roster of heroes they can trust to join them in their hours of need.
With a wide selection of Marvel characters to choose from, over the last 80 years the Fantastic Four has been home to many heroes. From street level characters like Spider-Man and Luke Cage, to Inhuman and Wakandan royalty and the Heralds of Galactus, the Fantastic Four has housed a wide variety of eclectic characters.