Writing A Custom Code Example

I have been stuck in a problem for a while now. I am trying to count the number of groups. I tried soo many things and nothing worked . Now I want to try to write a function that does the counting for me.

Can somebody write me an example of a cutom code that can be called from expression.
So my custome code should be something like that

Public int CountFun(int counter)
{
Return counter= counter +1;

}

Also how do I call this method from my expression?
i am going to call it from a field inside the groups, so each time the group is implemented i add one to the counter.

thanks