What should I do to get new content with Charlotte in studio? 

Go to the Living Room every Monday to catch her and Anna having conversation there and pick the right dialogues. Do that 3 times ( 3 Mondays ) and Charlotte will ask you to join her in her Yacht on Monday. 

When you're on her Yacht, pick the right dialogues and you'll get intimate with her. Do that 3 times ( 3 Mondays ) and when you go to the Studio, you'll be able to invite Charlotte to the Studio. 

I think it's important to pick the Anna route in the yatch when she asks you if you're with her, after that in another visit you'll get a threesome scene with Anna in the yatch, then after in a high level with Anna in the studio you can ask her "let's invite someone"