Grandpa is on the front row, second from the left. I'm thinking the man in the middle with the moustache may be their guide or minder; he looks nervous- as well one might be when one is dealing with wicked western capitalists and is answerable to Comrade Beria.
Here they are on the steps in front of the Cathedral of the Assumption. Grandpa is in the back row, second from the right.
And here they are next to the Tsar Bell- the biggest bell in the world- which fell to bits when it was being cast and has therefore never been rung. Grandpa is the figure in the middle, a step or two in front of everybody else.