Learn Polish: Przyjaciele piją razem piwo. - The friends are drinking beer together.

In this lesson on virile (male person) nouns, you will see many words taking endings like -i, -y, or -owie in the plural. However, przyjaciel (friend) is an exception. It takes the special irregular ending -ele to become przyjaciele.

How do I know to use piją for the verb?

The subject is przyjaciele (friends), which is a plural "they" subject. The verb pić (to drink) is a bit irregular, and its "they" form is piją.

Since the friends are drinking the beer, shouldn't piwo change its ending for the accusative case?

It is actually in the accusative case here! Because piwo is a neuter noun (it ends in -o), its accusative form looks exactly the same as its basic dictionary form.

Could I change the word order and say Przyjaciele razem piją piwo?

Yes, Polish word order is quite flexible. Putting razem (together) before the verb is completely fine and means the exact same thing, though keeping it after the verb is very natural.