"Let's eat!" is 먹자 in Korean.

"It's delicious " is 맛있네 or 맛있어 or 맛있다 in Korean. If you want to be more polite, you would say "맜있네요” or “맛있어요”.

If your friend is 20 years old, you can simply celebrate her by saying like "20살이 된걸 축하해".

"오늘같이" also means "오늘처럼". I guess 같이 has the same meaning as "처럼"which is "like" in English. 그녀는 바람같이 빨리 왔다. Or it has another meaning as "together" For example, "피자를 같이 먹었다." or "영화를 같이 봤다."

I think ‘맡다’ and ‘담당하다’ are similar to ‘take charge of’ or ‘take care of’ or ‘handle’ or ‘manage’ in English. ‘부담하다’ is like paying for something. For example, 요리는 제가 맡을게요. Or 요리는 제가 담당할게요. I will take care of cooking. 이 레스토랑에서 제 남자친구와 먹은 저녁은 제가 부담했어요. I paid for dinner with my boyfriend at this restaurant.

Join the company refers to starting to work at the company. Join to the company refers to connecting to the company.

'사람들로 복잡하다' - crowded with people이란 뜻인데 '~로' 또는 '~으로'는 영어로 'with'로 쓰여요. 다른 예를 들어보면, '컵에 물로 가득차 있다' The cup is filled with water라고 쓸수 있어요.