We know for a fact Dragon called for the Revolutionary leaders to meet up. And since Baltigo is destroyed, we don’t know whether the others actually arrived before the Blackbeard pirates. I am going to take a leap of faith and say they couldn’t get the chance to set off. Why? Because Dragon called them up personally and fixed another meeting point.
Where? The Holy Land Marijois.
That lady is going. Never has Dr. Kureha decided to butt into situations unrelated to her. So why is she going to the Reverie? Why go to an environment where she has to interact with a large number of people, even though she liked to stay alone?
Till now she has been in hiding, only to help others for something in return,only that one time where she decided to help Luffy and interact more than usual. She has an eccentric personality and not many things about her is clear. Not to mention an unusual life span. Another character has been shown to have some qualities as her: Ivankov. He had been in hiding, helped Luffy, eccentric personality, and was supposed to come out of hiding when Dragon ordered him to. I believe that is what Kureha is doing: following the order of the Revolutionary leader.
What would her role be? She is a doctor and vastly experienced due to her age. She could easily act as a tactical adviser/leader and a doctor.
*Theory by AWSUMSID