Shared functions do not require an instance of the class - that is the whole point. So to call one from within a different class you just prefix the function name with the name of the class it is declared in:
If the method is not declared as Shared, then you cannot call it without the appropriate instance of the class, either directly:
Dim mc As New MyClass
If you are outside the class it is declared in, or just by using the name of the method if you are in the same class, and not in a Shared method.