Destiny has said that if you have a friend that is in a clan, you can just be added to their private match. However! If you are in a clan, you can make your own.
They have said that you can go into a map solo even, but didn't say if you needed to be in clan or not.
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If that's what they said then it's safe to assume that in order to go in solo you'd need to be in a clan. Otherwise, how would you make a private match?