nextcloud/settings/ajax/createuser.php

<?php

OCP\JSON::callCheck();
OC_JSON::checkSubAdminUser();

$isadmin = OC_Group::inGroup(OC_User::getUser(), 'admin')?true:false;

if($isadmin) {
	$groups = array();
	if( isset( $_POST["groups"] )) {
		$groups = $_POST["groups"];
	}
}else{
	if(isset( $_POST["groups"] )) {
		$groups = array();
		foreach($_POST["groups"] as $group) {
			if(OC_SubAdmin::isGroupAccessible(OC_User::getUser(), $group)) {
				$groups[] = $group;
			}
		}
		if(count($groups) == 0) {
			$groups = OC_SubAdmin::getSubAdminsGroups(OC_User::getUser());
		}
	}else{
		$groups = OC_SubAdmin::getSubAdminsGroups(OC_User::getUser());
	}
}
$username = $_POST["username"];
$password = $_POST["password"];

// Does the group exist?
if( in_array( $username, OC_User::getUsers())) {
	OC_JSON::error(array("data" => array( "message" => "User already exists" )));
	exit();
}

// Return Success story
try {
	OC_User::createUser($username, $password);
	foreach( $groups as $i ) {
		if(!OC_Group::groupExists($i)) {
			OC_Group::createGroup($i);
		}
		OC_Group::addToGroup( $username, $i );
	}
	OC_JSON::success(array("data" =>
				array(
					"username" => $username,
					"groups" => implode( ", ", OC_Group::getUserGroups( $username )))));
} catch (Exception $exception) {
	OC_JSON::error(array("data" => array( "message" => $exception->getMessage())));
}