Just construct an object as if you were calling the constructor.
Code:
std::vector<cORGANISM> Dude; // Note no () here

Dude.push_back(cORGANISM());
If the constructor takes multiple arguments, then put them in there.

What it is most likely doing under the covers is creating a temporary object and then using the copy constructor to make a copy of that object and place it into the appropriate spot in the vector.