//First get all accounts on testrpc
 web3.eth.getAccounts((err, acc) => {    
    let accounts = acc;    
    contract.deployed().then(function(instance){            
      //you need to specify from and gas parameters
      instance.yourFunction(someParam, {from: accounts[0], gas: 500000});
    }).then(function(result) {
        console.log(result);
    });
  });