Is MENSA a global institution, and what benefits accrue from being a member?

Answer 1

Yes, Mensa has both an American Chapter, and many international chapters. For more information about the group, see Mensa.org

The benefits gained from the all volunteer group are entirely dependent on the amount of effort each member is willing to exercise. In other words, you get out of it according to what you put into it. Mensa works on the reciprocity principal.