It depends on the circumstances.
If you want to indicate an identity like "We are Mark and Luke" then you would use the verb SER. You would then say "nosotros somos Mark y Luke".
If you want to indicate a localisation or permanence in a place, like "We are in Rome" then you would say "nosotros estamos en Roma".
So, the verb "TO BE" has two different translations in Spanish depending on the context: The verb SER and the verb ESTAR. You can have a look at this video that explains the difference between these two a bit more clearly.
Remember that "nosotros" often gets omitted so in conversations and other circumstances people would only say "somos" or "estamos" without the need to use the pronoun "nosotros" (like, "somos Mark y Luke" or "estamos en Roma").
Hope this helps!
