In P2SH transactions, the address is the hash of the redeemScript. What is it for P2PKH transactions?
There are no "addresses" in protocol. There are only "output scripts". The address is only human-readable representation of script which matches the "output script".
The output script for bare multisig transaction is [m] + [hash1] + [hash2] + ... + [hashN] + [n]
And all of these parts are significant. There is no "classic" representation of such address yet.
bc.i uses the form of (Escrow X of X1 X2 X3) (where Xn is standard p2pkh address) but you may use your own.