Well, how is being a groupdefined? - Once you remember that, you can just step through the group axioms and show that they hold for (G,•) as well, provided they hold for (G,*).

For example: the associative property for (G,•) can be shown like this:

(a•b)•c = c*(b*a) = (c*b)*a = a•(b•c)

The first = holds, by definition of • in terms of *, the second = holds because (G,*) is a group, and the last = holds again by definition of • in terms of *.

Now you need to show the existence of a neutral element and of inverse elements.