Karass is a great term, and using it in conversation usually results in some knowing looks (plus some puzzled ones). First from the Urban Dictionary:
A group of people linked in a cosmically significant manner, even when superficial linkages are not evident.
And another take:
A karass is a spontaneously forming group, joined by unpredictable links, that actually gets stuff done— as Vonnegut describes it, "a team that do[es] God's Will without ever discovering what they are doing." A granfalloon, on the other hand, is a "false karass," a bureaucratic structure that looks like a team but is "meaningless in terms of the ways God gets things done."
No doubt you've experienced these two types of networks in your own life, many times over. The karass is that group of friends from college who have helped one another's careers in a hundred subtle ways over the years; the granfalloon is the marketing department at your firm, where everyone has a meticulously defined place on the org chart but nothing ever gets done. When you find yourself in a karass, it's an intuitive, unplanned experience. Getting into a granfalloon, on the other hand, usually involves showing two forms of ID.
karass: a term for a disparate group of people linked together without their knowledge. Your family and friends would not be part of your karass. You wouldn't choose its membership, and you may never know who is in it or what its purpose is.
I know that my karass includes various Ultrarunners, inlcuding the 3 mentioned yesterday. It truly is an adventure discovering other members along the trails.