If two groups are isomorphic, then they have the same cardinality. In order to show that two groups are isomorphic, you must display the isomorphism, that is, the conversion chart that shows how the elements of one group "translate" to the elements of the other group, and that every group operation checks out. Simply by listing the elements of each group, you can show that the groups have the same cardinality. It is also not difficult to show that both groups have an infinite number of members, if that is indeed the case.