Hello, here are some useful Maya tutorials:
Realistic eye modeling/texturing/rendering (the top 4 links)video tutorial.
however, i think only 1 person needs to model an eye and share the file, just change eye colour for each of our characters.
the basic idea, but can make changes to the shader network to make it look more interesting
There are 3 different techniques used in this tutorial, all effective. The most simple one is the Mia material which is practically just 1 button's click.
~YeenYee