Learn Finnish: Menkää yhdessä sisään!

Mennä is a type 3 verb. To form the plural or formal imperative, we take the infinitive stem men- and add the suffix -kää. We use -kää instead of -kaa because of vowel harmony, since the stem only contains the front vowel 'e'.

Does menkää mean we are talking to a group of people?

Usually, yes! The -kaa / -kää ending is the plural imperative used for telling two or more people to do something. However, it can also be used as a formal, polite command when speaking to just one person.

I see the word for "together" is yhdessä. Does this come from the number one (yksi)?

Exactly! Yhdessä is the inessive case form of the number yksi (meaning "in one"). So, Finnish literally expresses doing something "together" as doing it "in one".

Why does the sentence use sisään for "inside" instead of sisällä?

Finnish pays close attention to movement. Because the verb mennä (to go) implies movement toward or into a destination, you need the directional word sisään (into). You would use sisällä (inside / in) only if someone were already staying stationary inside.