Learn Icelandic: Fylgið okkur í húsið!

The -ið ending tells us this is a plural command, meaning you are speaking to two or more people. The singular command (speaking to one person) would be fylg.

Both the verb Fylgið and the noun húsið end in -ið. Is that the same ending?

No, they just happen to look exactly the same! On the verb Fylgið, the -ið is the plural command suffix. On the noun húsið, the -ið is the definite article attached to the end of the word, meaning 'the'.

Why is okkur (us) in the dative case here?

The verb fylgja (to follow) always requires its object to be in the dative case. The dative form of 'us' is okkur (which happily looks exactly the same as the accusative form, but the grammar rule here is strictly dative!).

Why do we say í húsið instead of í húsinu?

The preposition í can mean 'in' or 'into'. When it means 'into' and shows movement toward a new location, it requires the following word to be in the accusative case (húsið). If you were already resting inside the house, you would use the dative case (í húsinu).