I think it's all legit.

Note that you do not have to register an account with them in order to vote.

1) Select Groestlcoin from the list and click "Submit Payment 0.001 BTC"

2) On next screen type your name and email, then pick BTC, LTC or DOGE and press "Complete donation" button.

3) On the next screen you will see the wallet address to send donation.
NOTE: If sending money from exchanges (like Mintpal), be very careful that the amount is correct. Because they also change transaction fee.
I typed 0.001 BTC on Mintpal and didn't notice it, so actually only 0.0008 got sent (because 0.0002 is Mintpal transaction fee). So then I had to do another transaction.

Transfer BTC or LTC or DOGE (whichever you've selected) to that address.

4) Now you wait for email like this:

5) Click on the link. Only now your vote is counted.

Thank you!