How can I add a string to a vector string?

If you are new to C++ I strongly recommend reading a book for beginners or attending online video course. But it would look something similar to:

#include<vector>
#include<string>

class Party{
   std::vector<std::string> m_members;

public:
   void add_new_player (const std::string& val){
      m_members.push_back(val);
   }

   // Add remaining functionality here...
};