This is the 713th most frequent Ukrainian word.
сі
The Ukrainian word "сі" translates to "these" in English.
In this sentence, 'сі' functions as an attributive demonstrative pronoun, meaning 'these' and describing the noun 'люди' (people).
Сі люди прийшли до міста сьогодні.
These people came to the city today.
Here, 'сі' is used reflexively in combination with 'посіятися', conveying an idiomatic expression about speaking loudly and clearly.
Він полюбляє сі посіятися мов дзвін.
He loves to spread his voice like a bell.
Similarly as in the first example, 'сі' modifies the noun 'вечірки' (parties), serving the same demonstrative function.