>>string user_full_name = user_first_name + " " + user_last_name;
>>cout << "Please enter your first name: ";
>>cin >> user_first_name;
Also, clearly, there is no point in putting together the full name before you've actually collected the first and last names.
Remember: code is executed sequentially, so only before your cout statement that prints the full name do you have the first and last names stored in your variables.