How likely is it that all the addresses involved in a SINGLE multisig transaction input/output belong to the same wallet/owner? I read from a paper that: "Since many transactions have multiple sending addresses, we can make the reasonable assumption that all these addresses have the same owner." (Quantitative Analysis of the Full Bitcoin Transaction Graph by Dorit Ron and Adi Shamir) So I wonder if the same could be said about the addresses taking part in a multisignature of a multisig transaction input/output.
Multisig is useful for both a single individual and for a group. Individuals can use it to secure their funds by using a 2/3 address and distributing the keys. Groups can use multisig to implement an escrow transaction for goods or services. 

These, and other use-cases, for multisig are being used today, but it's impossible to know the proportion of individual-use and group-use. There is no way to know for certain if they are controlled by the same person, unless you have more information than just what is available in the blockchain.