Does this sentence sound natural? "드디어... 오늘 언니들과 (친구이름)와 함께 (이벤트이름)에서! 이따가 저희 무대 기대해주세요"
Trying to say: "Finally... at (event name) today with my eonnis and (friend's name)! Please anticipate our performance later."

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Hello.
I'm a native Korean speaker.

Your writing reads natural.
If I make a minor comment, 이따가 is not the formal writing. The formal one is 있다가.

Hope this helps.:)

Answer

드디어..오늘 ~에서 언니들과 (친구이름)과 함께! 
(Your last sentence is fine but if you want to make it sound formal, 잠시 후에 있을 저희 무대 기대해주세요.)