This is the 99th most frequent Dutch word.
allemaal
"All," "everyone," or "everything."
Here, 'allemaal' is used to address everybody in the group, emphasizing that everyone came.
Wat leuk dat jullie allemaal gekomen zijn.
How nice that you all came.
In this case, 'allemaal' emphasizes the entire statement or situation being untrue.
Dat is allemaal niet waar.
That is all not true.
In this sentence, 'allemaal' is used to mean 'all' or 'everyone,' referring to the collective group going to the party.