Re: function can't change a pointer's address

I've suddenly become quite stupid.
I'm passing a pointer to a function that can't change the pointer's address.
Can someone tell me why this is happening?

//I have a list of cGroup objects that I access using pointers
cGroup *pg;
pg=GetGroup(editPoly); //just returns a pointer to the group a polygon
belongs to
ChooseGroup(pg); //opens a dialog that displays the current group
and allows you to choose another group
//When ChooseGroup() returns pg hasn't changed!

cGroup *ChooseGroup(cGroup *pg){
    INT_PTR ret;
    if(ret=DialogBoxParam((HINSTANCE)ghInstance,
                                        MAKEINTRESOURCE(IDD_CHOOSEGROUP),
                                        ghWnd,(DLGPROC)ChooseGroupProc,
                                        (LPARAM)pg)) //pg is passed here
just to display the current group in the dialog box
    {
        //at this point pg's address is the same as was passed in the
argument list
        //and it is the same as it was before calling this function
        pg=DlgGroup; //cGroup *DlgGroup is a file scope variable used to
record the choice of group in ChooseGroupProc
        return pg; //pg's address has changed here but returns to
it's previous address after exiting the function.
    }
    else
    return 0;
}

I thought I understood this stuff but it looks as though a few brain cells
have gone missing.
This should be simple but I can't see it.