Interface Group

A group object whose members and those members' roles within the group can be queried.

Here's an example which shows the members of a group. Before running it, replace the email address of the group with that of one on your domain.

function listGroupMembers() {
  var group = GroupsApp.getGroupByEmail("example@googlegroups.com");
  var str = group.getEmail() + ': ';
  var users = group.getUsers();
  for (var i = 0; i < users.length; i++) {
    var user = users[i];
    str = str + user.getEmail() + ", ";
  }
  Logger.log(str);
}