How do you set a users group?
It's not documented, but UserGroupsService has a method assignUserToGroups($userId, int|array $groupIds). You'll have to save your user first, but once you have a user id, I believe you can just use that with something like this:

// Original code
$user = new UserModel;

$user-&gt;username = 'username';
$user-&gt;email = 'user@email.com';
$user-&gt;firstName = 'first name';
$user-&gt;lastName = 'last name';
$user-&gt;getContent()-&gt;setAttributes([ 'usersAddress' =&gt; 'Your face' ]);

craft()-&gt;users-&gt;saveUser($user);

// Add to single group
craft()-&gt;userGroups-&gt;assignUserToGroups($user-&gt;id, $groupId);

// OR, add to multiple groups
craft()-&gt;userGroups-&gt;assignUserToGroups($user-&gt;id, [$groupIdOne, $groupIdTwo]);