How many people own crypto currencies?
As Luca mentioned it is really difficult to measure, but there is a way to map Bitcoin addresses to IP address under certain conditions.
Refer this paper

Title : An Analysis of Anonymity in Bitcoin using P2P Network Traffic. 

Authors : Philip Koshy, Diana Koshy, and Patrick McDaniel.

Note: It analyses relay pattern of transactions. It cannot analyse transactions that are already confirmed in blocks. 

Some companies publicly disclose their data and it might be possible to come up with a ballpark estimate.  Blockchain.info shares the number of wallets used in their My Wallet service.