"Let's invite them to our home in Poland!"
Translation:Ni invitu ilin al nia hejmo en Pollando!
Why isn't the accusative being used? Why isn't it "al nian hejmon in Polando"? There is a movement to the house, isn't there?
Accusative is only used after those prepositions that can indicate either movement or location, to show that it's the movement which is meant.
al (and ĝis and tra) always indicate movement, so they don't need to distinguish the two cases and so they always take the nominative case, as is the general rule with prepositions.