This is the 1558th most frequent Polish word.
dołączył
"Dołączył" means "joined" in English.
The word 'dołączył' is used here to indicate someone joining or participating in a group activity.
On dołączył do nas wczoraj na kolację.
He joined us yesterday for dinner.
In this context, 'dołączył' is applied in the sense of attaching an item (the file) to something else (the email).
Dołączył nowy plik do e-maila, żeby wysłać wszystkie dokumenty naraz.
He attached a new file to the email to send all documents at once.
Here, 'dołączył' expresses someone becoming a member of a group or organization.