Comic Book Questions Answered – where I answer whatever questions you folks might have about comic books (feel free to e-mail questions to me at firstname.lastname@example.org).
Reader Andrew wrote in to ask, "I wanted to know, has an inhuman ever been to Xavier's school, or been an xmen? Or a non-mutant hero as an Xman? Thanks."
First, let's address your second question first. The X-Men do not, in fact, insist on you being a mutant for you to be a member of the X-Men. I mean, it's generally a good idea, but it is not a requirement. A couple of the most notable non-mutant members of the X-Men are Longshot, who is an alien, and the Juggernaut, who is a human who was given a mystic ruby that gave him super-powers. Those aren't the only ones, of course, but they are the most notable examples.
Amusingly enough, Longshot's sort of induction into the team (along with a pair of other new recruits, Psylocke and Dazzler) came in a fight WITH the Juggernaut!
Some of the other ones carry with them major caveats, like how they have had a number of members who were FORMER mutants or stuff like Warlock, who is an alien, but you could argue that he is a MUTANT alien (similarly, one of their earliest human members, Mimic, was later revealed to have been a mutant the entire time).
So, yes, Andrew, there are non-mutant heroes in the X-Men.
As for Inhumans, that is a different story.
You probably already know that the X-Men and the Inhumans had a little problem a while back when it was revealed that the Terrigenesis Mists (which Black Bolt had released into Earth's atmosphere) were now deadly to mutants. So the X-Men and the Inhumans actually ended up having a sort of war between the two groups...
So, in a war between superheroes, it would be logical enough if there were heroes who were caught between the two sides, Romeo and Juliet style, but in this case, the hero who was sort of in that position was the REVERSE. Beast, longtime member of the X-Men, had not joined a new team in a while (he has been a member of both the Defenders and the Avengers) so he decided to join up with the Inhumans to work on the Terrigen Mists to see if he couldn't find a cure.
Of course, in a fight between the X-Men and the Inhumans, Beast ultimately had to side with the X-Men.
There HAVE been a number of Inhumans who once believed that they were mutants. So that opens up to the possibility that there were members of the X-Men.