This is the 1925th most frequent Polish word.
wszystkich
"All" or "of all" (genitive plural form).
In this example, 'wszystkim' is actually the dative plural form of 'wszyscy,' used to indicate the indirect object of the gratitude.
Dziękuję wszystkim za pomoc.
I thank everyone for their help.
The word 'wszystkich' is used as the accusative plural form of 'wszyscy,' taking the role of a direct object in the sentence to signify 'everyone' invited.
Zaprosiłem wszystkich na przyjęcie.
I invited everyone to the party.
Here, 'wszystkich' is the plural genitive form of 'wszyscy,' used as the object of the action implied by 'zaskoczyła.'