Integrate their changes and extend data structures

Extending a project from version 1.0 which has full feature set. Everything in place. Now would like to extend this project with new dimension. Let say there is a structure,

struct person {
std::string name;
uint32_t id;
uint8_t age;
uint8_t sex;
};
std::list<struct person> people;

So, people list has all the persons. Now based on the employee category would like to extend above structure as below.

#define EMP_CAT uint32_t
std::map<EMP_CAT, people> division_people;

Now division_people map holds persons based on their employee division type like doctors, nurses, admins etc;

Since this is a huge code, the project may take couple of months to extend this way. Mean while other developers keep work on extending other features with existed data structures only.

Can someone suggest better way to merge the other developers continuous integration changes and keep doing the changes required to complete the above project?