。
Everyone who came was welcomed.
Literal
Come-[attributive-た] people [topic-は] all were-welcomed.
来た人たち is a relative clause: the past plain 来た modifies 人たち ('the people') as 'people who came.' 歓迎された is the passive past of 歓迎する ('to welcome'), keeping the sentence subject-focused on the welcomed party. 皆 (read みな here) is 'everyone / all.'