This is the 4475th most frequent Polish word.
kolegi
"Friends" or "colleagues" (genitive or nominative plural of "kolega").
The word 'kolegi' is used here in the genitive case, indicating possession or association with the object 'na przyjęcie' (to the party).
Poszliśmy razem do kolegi na przyjęcie.
We went together to a friend's party.
The word 'kolegi' is again in the genitive case, here denoting possession or association related to 'u mojego' (at my).
Widziałem ten samochód u mojego kolegi.
I saw this car at my friend's place.
Here, 'kolegi' functions in the genitive case representing possession, connected to 'zeszytu' (notebook).